Pull buildhasher out of green::Builder's maps

[CAD97]

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bors: r+
🤖

[codecov]

Codecov Report

Merging #41 into master will decrease coverage by 0.93%.
The diff coverage is 70.96%.

@@            Coverage Diff             @@
##           master      #41      +/-   ##
==========================================
- Coverage   79.00%   78.07%   -0.94%     
==========================================
  Files           8        8              
  Lines         543      561      +18     
==========================================
+ Hits          429      438       +9     
- Misses        114      123       +9     

Impacted Files	Coverage Δ
src/green/builder.rs	60.57% <70.96%> (-2.22%)	⬇️

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[bors]

Build succeeded:

• Clippy
• Miri
• Rustfmt
• Tests