add method on object CommutativeApplicative to get CommutativeMonoid

[alexeygorobets]

Close #2716

[codecov-io]

Codecov Report

Merging #2747 into master will increase coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #2747      +/-   ##
==========================================
+ Coverage   95.33%   95.34%   +<.01%     
==========================================
  Files         363      363              
  Lines        6777     6782       +5     
  Branches      299      290       -9     
==========================================
+ Hits         6461     6466       +5     
  Misses        316      316

Impacted Files	Coverage Δ
core/src/main/scala/cats/CommutativeApply.scala	66.66% <100%> (+66.66%)	⬆️
...e/src/main/scala/cats/CommutativeApplicative.scala	75% <100%> (+75%)	⬆️

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[kailuowang]

Thanks!