Optimize FreeApplicative.product

[peterneyens]

Follow up after #1748

Ap now contains two FreeApplicatives (instead of just one before), so we can do some more optimisation as we are currently doing. (The new implementation should also result in the same FreeApplicative as the original implementation).

Currently the result of FreeApplicative's map2 is one of the following two :

Pure(f(a, b))
Ap({ case (a, b) => f(a, b) }, Ap(Ap(Pure(a => b => (a, b)), fa), fb))

With this change it can be one of :

Pure(f(a, b))
Ap(Pure(f(a, _)), fb)
Ap(Pure(f(_, b)), fa)
Ap(Ap(Pure(a => b => f(a, b))), fa), fb)

[codecov-io]

Codecov Report

Merging #1921 into master will increase coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #1921      +/-   ##
==========================================
+ Coverage   95.54%   95.55%   +<.01%     
==========================================
  Files         248      248              
  Lines        4426     4430       +4     
  Branches      116      117       +1     
==========================================
+ Hits         4229     4233       +4     
  Misses        197      197

Impacted Files	Coverage Δ
...ree/src/main/scala/cats/free/FreeApplicative.scala	100% <100%> (ø)	⬆️

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[edmundnoble]

This is awesome, it's causing less stack jumps in https://github.com/typelevel/cats/blob/master/free/src/main/scala/cats/free/FreeApplicative.scala#L99 by creating more single-argument functions. 👍 from me.

[edmundnoble]

/cc @safareli