add evaluable functor: atan2

[ichiban]

#144

[codecov-commenter]

Codecov Report

Merging #170 (fa82d26) into main (e605e4c) will increase coverage by 0.01%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main     #170      +/-   ##
==========================================
+ Coverage   95.45%   95.46%   +0.01%     
==========================================
  Files          19       19              
  Lines        6005     6026      +21     
==========================================
+ Hits         5732     5753      +21     
  Misses        228      228              
  Partials       45       45              

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

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