Replace the build script with a codegent crate

[Ogeon]

This lets us generate code in advance, rather than on demand, removing a step from the build process.

Related to #416.

[codspeed-hq]

CodSpeed Performance Report

Merging #417 will improve performances by 10.14%

_{Comparing codegen (a423de9) with master (dcc802a)}

Summary

⚡ 1 improvements
✅ 46 untouched benchmarks

Benchmarks breakdown

	Benchmark	master	codegen	Change
⚡	multiply_3x3	316.7 ns	287.5 ns	+10.14%

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 82.87%. Comparing base (dcc802a) to head (a423de9).
Report is 2 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #417      +/-   ##
==========================================
- Coverage   82.90%   82.87%   -0.04%     
==========================================
  Files         132      130       -2     
  Lines       20260    20199      -61     
  Branches    20260    20199      -61     
==========================================
- Hits        16796    16739      -57     
  Misses       3337     3337              
+ Partials      127      123       -4     

Components	Coverage Δ
palette	82.93% <100.00%> (-0.04%)	⬇️
palette_derive	81.98% <ø> (ø)