add evaluable functor: acos

[ichiban]

#144

[codecov-commenter]

Codecov Report

Merging #169 (a54535a) into main (1358382) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##             main     #169   +/-   ##
=======================================
  Coverage   95.44%   95.45%           
=======================================
  Files          19       19           
  Lines        5992     6005   +13     
=======================================
+ Hits         5719     5732   +13     
  Misses        228      228           
  Partials       45       45           

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

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