Demonstrate Columnar batch builder

[frankmcsherry]

The columnar.rs example is extended to use columnar containers for the merge batcher, allowing most of the merge batcher state to be in serialized form. The runtimes of the example are so

     Running `target/release/examples/columnar 1000000 1000`
3.344818375s    loading complete
7.658054084s    queries complete
7.662470667s    shut down

versus the current spines.rs example

     Running `target/release/examples/spines 1000000 1000 new`
Running ["new"] arrangement
2.845316875s    loading complete
6.4664785s      queries complete
6.469992416s    shut down

[antiguru]

Looks good!

[antiguru]

The last commit restores performance by producing a key batch instead of a val batch:

➜  differential-dataflow git:(columnar_builder) cargo run --release --example spines -- 1000000 1000 new
   Compiling differential-dataflow v0.15.3 (/home/moritz/dev/repos/differential-dataflow/differential-dataflow)
    Finished `release` profile [optimized + debuginfo] target(s) in 38.86s
     Running `target/release/examples/spines 1000000 1000 new`
Running ["new"] arrangement
4.410642495s    loading complete
10.006482626s   queries complete
10.012127944s   shut down
➜  differential-dataflow git:(columnar_builder) cargo run --release --example columnar -- 1000000 1000
    Finished `release` profile [optimized + debuginfo] target(s) in 0.03s
     Running `target/release/examples/columnar 1000000 1000`
4.089337934s    loading complete
9.359655481s    queries complete
9.365747091s    shut down