add evaluable functor: integer power

[ichiban]

#144

[codecov-commenter]

Codecov Report

Merging #167 (ce3603f) into main (c7d6a93) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##             main     #167   +/-   ##
=======================================
  Coverage   95.42%   95.43%           
=======================================
  Files          19       19           
  Lines        5967     5979   +12     
=======================================
+ Hits         5694     5706   +12     
  Misses        228      228           
  Partials       45       45           

Impacted Files	Coverage Δ
engine/number.go	100.00% <100.00%> (ø)

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