Add Commutative{Monad, FlatMap} instances for IdT

[ceedubs]

No description provided.

[codecov-io]

Codecov Report

Merging #2104 into master will increase coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #2104      +/-   ##
==========================================
+ Coverage   94.79%   94.79%   +<.01%     
==========================================
  Files         325      325              
  Lines        5490     5492       +2     
  Branches      214      221       +7     
==========================================
+ Hits         5204     5206       +2     
  Misses        286      286

Impacted Files	Coverage Δ
core/src/main/scala/cats/data/IdT.scala	97.72% <100%> (+0.1%)	⬆️

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[LukaJCB]

Looks great, thanks! :)