implement ProjectTo for NamedDimsArray

[mzgubic]

Closes #200

[oxinabox]

The requirements for this are.

• Projecting conflicting names throws an error
• projecting unnamed trangent (which is an abstract array) onto named primal gets primals name
• projecting partially unnamed tangent onto fully named primal gets primal name
• projecting fully named tangent onto partially named primal gets fully named
• projecting (:_, :B) onto (:A, _) gets (:A, :B)

I would like to see these in the tests

[mzgubic]

Ah, good catch. I copied it over from AxisKeys which is similarly cavalier about it, will open an issue there to track it.

[codecov]

Codecov Report

Merging #201 (e22cfdc) into master (712a901) will increase coverage by 0.04%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #201      +/-   ##
==========================================
+ Coverage   94.79%   94.84%   +0.04%     
==========================================
  Files          14       14              
  Lines         673      679       +6     
==========================================
+ Hits          638      644       +6     
  Misses         35       35              

Impacted Files	Coverage Δ
src/chainrules.jl	92.85% <100.00%> (+5.35%)	⬆️

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[oxinabox]

Yes, this is good