Improve performance of hashing

[JosephDuffy]

Roughly 2x faster by removing the need to store closures. Maybe there's more that can be done here.

I also tried doing similar changes to equality, but this would require evaluating all key paths for equality, or KeyPath<Root, some Hashable> to be compatible with e.g. KeyPath<Root, String>.

[codecov]

Codecov Report

Base: 99.45% // Head: 99.74% // Increases project coverage by +0.29% 🎉

Coverage data is based on head (21f9a9a) compared to base (f9e6d78).
Patch coverage: 97.16% of modified lines in pull request are covered.

❗ Current head 21f9a9a differs from pull request most recent head 182a6e7. Consider uploading reports for the commit 182a6e7 to get more accurate results

Additional details and impacted files

@@            Coverage Diff             @@
##           master      #33      +/-   ##
==========================================
+ Coverage   99.45%   99.74%   +0.29%     
==========================================
  Files          15       16       +1     
  Lines        3507     3585      +78     
==========================================
+ Hits         3488     3576      +88     
+ Misses         19        9      -10     

Impacted Files	Coverage Δ
...s/HashableByKeyPath/HashableKeyPathForwarder.swift	66.66% <ø> (ø)
Sources/HashableByKeyPath/KeyPathHasher.swift	70.00% <70.00%> (ø)
Sources/HashableByKeyPath/HashableByKeyPath.swift	100.00% <100.00%> (ø)
...ashableByKeyPathTests/HashableByKeyPathTests.swift	100.00% <100.00%> (ø)

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