WIP: ProjectTo implementation

[mzgubic]

Building on #385. Alternative to #380, #382 and #383. See usage in JuliaDiff/ChainRules.jl#459

[codecov-commenter]

Codecov Report

❌ Patch coverage is 90.90909% with 5 lines in your changes missing coverage. Please review.
✅ Project coverage is 89.35%. Comparing base (653cd7d) to head (eefd84f).

Files with missing lines	Patch %	Lines
src/projection.jl	90.90%	5 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #387      +/-   ##
==========================================
+ Coverage   89.20%   89.35%   +0.15%     
==========================================
  Files          14       15       +1     
  Lines         565      620      +55     
==========================================
+ Hits          504      554      +50     
- Misses         61       66       +5     

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