events: optimize once() and removeListener()

[mcollina]

Optimize the EventEmitter hot paths:

• rewrite the once() wrapper as a closure instead of bind() + a state object
• keep the _events object shape in removeListener(): store undefined instead of delete-ing keys or reallocating the object (eventNames() now filters, matching Stream.prototype.eventNames); removes the now-unused kShapeMode
• outline the unhandled error path from emit() (439 → 278 bytecodes) and the max-listeners warning from _addListener() (405 → 288) so both fit within the V8 inlining budgets

benchmark/compare.js --runs 30 before/after all changes:

events

                                                                 confidence improvement accuracy (*)   (**)  (***)
events/ee-add-remove.js n=1000000 removeListener=0 newListener=0        ***     12.59 %       ±1.59% ±2.11% ±2.76%
events/ee-add-remove.js n=1000000 removeListener=0 newListener=1          *      2.37 %       ±1.92% ±2.58% ±3.40%
events/ee-add-remove.js n=1000000 removeListener=1 newListener=0        ***     15.12 %       ±1.14% ±1.52% ±1.99%
events/ee-add-remove.js n=1000000 removeListener=1 newListener=1        ***     21.82 %       ±2.24% ±3.01% ±3.97%
events/ee-emit.js listeners=1 argc=0 n=2000000                                   0.89 %       ±3.72% ±4.95% ±6.47%
events/ee-emit.js listeners=1 argc=10 n=2000000                                 -0.85 %       ±2.46% ±3.28% ±4.28%
events/ee-emit.js listeners=1 argc=2 n=2000000                                   1.25 %       ±3.73% ±4.97% ±6.50%
events/ee-emit.js listeners=1 argc=4 n=2000000                                  -0.11 %       ±2.91% ±3.87% ±5.04%
events/ee-emit.js listeners=10 argc=0 n=2000000                                  0.99 %       ±1.95% ±2.60% ±3.38%
events/ee-emit.js listeners=10 argc=10 n=2000000                                -0.24 %       ±1.64% ±2.18% ±2.84%
events/ee-emit.js listeners=10 argc=2 n=2000000                                  0.05 %       ±1.61% ±2.14% ±2.79%
events/ee-emit.js listeners=10 argc=4 n=2000000                          **      2.22 %       ±1.60% ±2.13% ±2.77%
events/ee-emit.js listeners=5 argc=0 n=2000000                                   1.82 %       ±2.04% ±2.72% ±3.54%
events/ee-emit.js listeners=5 argc=10 n=2000000                                 -0.26 %       ±1.61% ±2.14% ±2.79%
events/ee-emit.js listeners=5 argc=2 n=2000000                            *     -2.25 %       ±1.96% ±2.61% ±3.39%
events/ee-emit.js listeners=5 argc=4 n=2000000                                   0.64 %       ±2.11% ±2.81% ±3.66%
events/ee-listen-unique.js n=1000000 events=1                           ***     14.85 %       ±0.80% ±1.06% ±1.39%
events/ee-listen-unique.js n=1000000 events=10                          ***      8.46 %       ±1.28% ±1.71% ±2.23%
events/ee-listen-unique.js n=1000000 events=2                           ***     14.09 %       ±0.88% ±1.18% ±1.54%
events/ee-listen-unique.js n=1000000 events=20                          ***      7.53 %       ±1.38% ±1.84% ±2.40%
events/ee-listen-unique.js n=1000000 events=3                           ***      9.24 %       ±1.83% ±2.44% ±3.20%
events/ee-listen-unique.js n=1000000 events=5                           ***     13.22 %       ±1.39% ±1.86% ±2.45%
events/ee-listener-count-on-prototype.js n=50000000                              0.14 %       ±0.82% ±1.09% ±1.41%
events/ee-listeners.js raw='false' listeners=5 n=5000000                ***     -3.03 %       ±1.73% ±2.31% ±3.03%
events/ee-listeners.js raw='false' listeners=50 n=5000000                       -0.00 %       ±0.54% ±0.72% ±0.94%
events/ee-listeners.js raw='true' listeners=5 n=5000000                 ***      5.87 %       ±2.69% ±3.59% ±4.68%
events/ee-listeners.js raw='true' listeners=50 n=5000000                        -0.14 %       ±1.05% ±1.40% ±1.83%
events/ee-once.js argc=0 n=20000000                                     ***     25.71 %       ±1.08% ±1.43% ±1.87%
events/ee-once.js argc=1 n=20000000                                     ***     24.94 %       ±0.77% ±1.02% ±1.33%
events/ee-once.js argc=4 n=20000000                                     ***     25.14 %       ±0.92% ±1.23% ±1.61%
events/ee-once.js argc=5 n=20000000                                     ***     25.63 %       ±0.90% ±1.21% ±1.58%

Be aware that when doing many comparisons the risk of a false-positive result increases.
In this case, there are 31 comparisons, you can thus expect the following amount of false-positive results:
  1.55 false positives, when considering a   5% risk acceptance (*, **, ***),
  0.31 false positives, when considering a   1% risk acceptance (**, ***),
  0.03 false positives, when considering a 0.1% risk acceptance (***)

streams

                                                                                         confidence improvement accuracy (*)    (**)   (***)
streams/creation.js kind='duplex' n=50000000                                                    ***      1.32 %       ±0.61%  ±0.81%  ±1.05%
streams/creation.js kind='readable' n=50000000                                                          -0.37 %       ±0.61%  ±0.81%  ±1.06%
streams/creation.js kind='transform' n=50000000                                                   *      3.83 %       ±3.24%  ±4.37%  ±5.78%
streams/creation.js kind='writable' n=50000000                                                  ***      2.83 %       ±0.55%  ±0.73%  ±0.95%
streams/destroy.js kind='duplex' n=1000000                                                              -0.08 %       ±0.64%  ±0.85%  ±1.11%
streams/destroy.js kind='readable' n=1000000                                                    ***      3.98 %       ±1.82%  ±2.42%  ±3.16%
streams/destroy.js kind='transform' n=1000000                                                            0.32 %       ±0.54%  ±0.72%  ±0.94%
streams/destroy.js kind='writable' n=1000000                                                    ***      2.57 %       ±0.60%  ±0.79%  ±1.03%
streams/iter-creation.js n=100000 type='pair' api='classic'                                              1.49 %       ±4.62%  ±6.15%  ±8.00%
streams/iter-creation.js n=100000 type='pair' api='iter'                                                 0.02 %       ±0.78%  ±1.04%  ±1.35%
streams/iter-creation.js n=100000 type='pair' api='webstream'                                           -0.31 %       ±1.61%  ±2.14%  ±2.78%
streams/iter-creation.js n=100000 type='readable' api='classic'                                         -0.15 %       ±5.01%  ±6.67%  ±8.69%
streams/iter-creation.js n=100000 type='readable' api='iter'                                             0.48 %       ±2.18%  ±2.90%  ±3.77%
streams/iter-creation.js n=100000 type='readable' api='webstream'                                       -0.29 %       ±1.71%  ±2.27%  ±2.96%
streams/iter-creation.js n=100000 type='transform' api='classic'                                         1.97 %       ±2.41%  ±3.21%  ±4.18%
streams/iter-creation.js n=100000 type='transform' api='webstream'                                       0.19 %       ±0.86%  ±1.14%  ±1.49%
streams/iter-creation.js n=100000 type='writable' api='classic'                                          1.07 %       ±2.10%  ±2.80%  ±3.64%
streams/iter-creation.js n=100000 type='writable' api='iter'                                             0.60 %       ±1.89%  ±2.51%  ±3.27%
streams/iter-creation.js n=100000 type='writable' api='webstream'                                       -0.18 %       ±1.23%  ±1.64%  ±2.13%
streams/pipe.js n=5000000                                                                       ***     13.92 %       ±1.41%  ±1.88%  ±2.45%
streams/readable-async-iterator.js sync='no' n=100000                                             *      2.45 %       ±1.86%  ±2.48%  ±3.24%
streams/readable-async-iterator.js sync='yes' n=100000                                                  -3.12 %       ±3.85%  ±5.15%  ±6.76%
streams/readable-bigread.js n=1000                                                                       0.14 %       ±2.98%  ±3.96%  ±5.16%
streams/readable-readall.js n=5000                                                                      -0.24 %       ±2.49%  ±3.32%  ±4.33%
streams/writable-manywrites.js len=1024 callback='no' writev='no' sync='no' n=100000                    -0.87 %       ±2.73%  ±3.64%  ±4.75%
streams/writable-manywrites.js len=1024 callback='no' writev='no' sync='yes' n=100000                    1.20 %       ±4.18%  ±5.57%  ±7.29%
streams/writable-manywrites.js len=1024 callback='no' writev='yes' sync='no' n=100000                    2.47 %       ±5.23%  ±6.98%  ±9.13%
streams/writable-manywrites.js len=1024 callback='no' writev='yes' sync='yes' n=100000                  -1.24 %       ±7.14%  ±9.50% ±12.36%
streams/writable-manywrites.js len=1024 callback='yes' writev='no' sync='no' n=100000             *      2.65 %       ±2.43%  ±3.23%  ±4.22%
streams/writable-manywrites.js len=1024 callback='yes' writev='no' sync='yes' n=100000           **      8.29 %       ±5.26%  ±7.00%  ±9.11%
streams/writable-manywrites.js len=1024 callback='yes' writev='yes' sync='no' n=100000                   2.01 %       ±4.22%  ±5.62%  ±7.31%
streams/writable-manywrites.js len=1024 callback='yes' writev='yes' sync='yes' n=100000           *      5.16 %       ±5.15%  ±6.85%  ±8.92%
streams/writable-manywrites.js len=32768 callback='no' writev='no' sync='no' n=100000                    2.67 %       ±3.56%  ±4.73%  ±6.16%
streams/writable-manywrites.js len=32768 callback='no' writev='no' sync='yes' n=100000            *      5.65 %       ±4.82%  ±6.42%  ±8.37%
streams/writable-manywrites.js len=32768 callback='no' writev='yes' sync='no' n=100000                  -0.76 %       ±1.95%  ±2.59%  ±3.37%
streams/writable-manywrites.js len=32768 callback='no' writev='yes' sync='yes' n=100000                  4.26 %       ±8.13% ±10.81% ±14.07%
streams/writable-manywrites.js len=32768 callback='yes' writev='no' sync='no' n=100000                  -1.57 %       ±3.42%  ±4.56%  ±5.94%
streams/writable-manywrites.js len=32768 callback='yes' writev='no' sync='yes' n=100000                  2.83 %       ±4.44%  ±5.91%  ±7.69%
streams/writable-manywrites.js len=32768 callback='yes' writev='yes' sync='no' n=100000                 -0.87 %       ±2.98%  ±3.96%  ±5.16%
streams/writable-manywrites.js len=32768 callback='yes' writev='yes' sync='yes' n=100000                 2.03 %       ±5.96%  ±7.93% ±10.32%

Be aware that when doing many comparisons the risk of a false-positive result increases.
In this case, there are 40 comparisons, you can thus expect the following amount of false-positive results:
  2.00 false positives, when considering a   5% risk acceptance (*, **, ***),
  0.40 false positives, when considering a   1% risk acceptance (**, ***),
  0.04 false positives, when considering a 0.1% risk acceptance (***)

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 90.24%. Comparing base (1d87a24) to head (ce6c9ad).
⚠️ Report is 13 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #64373      +/-   ##
==========================================
- Coverage   90.25%   90.24%   -0.01%     
==========================================
  Files         741      741              
  Lines      240990   241196     +206     
  Branches    45402    45431      +29     
==========================================
+ Hits       217502   217666     +164     
- Misses      15066    15086      +20     
- Partials     8422     8444      +22     

Files with missing lines	Coverage Δ
lib/events.js	99.59% <100.00%> (+<0.01%)	⬆️

... and 48 files with indirect coverage changes

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