add evaluable functor: div

[ichiban]

#144

[codecov-commenter]

Codecov Report

Merging #158 (7c34f19) into main (6d174c7) will increase coverage by 0.02%.
The diff coverage is 81.11%.

@@            Coverage Diff             @@
##             main     #158      +/-   ##
==========================================
+ Coverage   93.99%   94.02%   +0.02%     
==========================================
  Files          18       19       +1     
  Lines        5361     5385      +24     
==========================================
+ Hits         5039     5063      +24     
  Misses        272      272              
  Partials       50       50              

Impacted Files	Coverage Δ
engine/builtin.go	92.84% <ø> (+1.45%)	⬆️
engine/eval.go	81.11% <81.11%> (ø)

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