Intern build keys

[samuelcolvin]

Based off #177 can be rebased once that's merged.

This makes building the complete benchmark model ~30% faster.

[codecov]

Codecov Report

Merging #178 (ef5856b) into main (f29eec1) will increase coverage by 0.18%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main     #178      +/-   ##
==========================================
+ Coverage   97.58%   97.77%   +0.18%     
==========================================
  Files          45       45              
  Lines        4391     4451      +60     
  Branches       32       32              
==========================================
+ Hits         4285     4352      +67     
+ Misses        106       99       -7     

Impacted Files	Coverage Δ
src/build_tools.rs	96.92% <100.00%> (+11.20%)	⬆️
src/validators/bytes.rs	96.77% <100.00%> (+0.16%)	⬆️
src/validators/date.rs	95.12% <100.00%> (+0.06%)	⬆️
src/validators/datetime.rs	96.00% <100.00%> (+0.08%)	⬆️
src/validators/dict.rs	95.69% <100.00%> (+0.04%)	⬆️
src/validators/float.rs	98.76% <100.00%> (+0.03%)	⬆️
src/validators/function.rs	96.33% <100.00%> (+0.09%)	⬆️
src/validators/int.rs	100.00% <100.00%> (ø)
src/validators/is_instance.rs	96.42% <100.00%> (ø)
src/validators/list.rs	100.00% <100.00%> (ø)
... and 18 more

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