Backport #2742 Add separateFoldable to Alternative

[gagandeepkalra]

addresses #3143

[codecov-io]

Codecov Report

Merging #3341 into scala_2.11 will increase coverage by <.01%.
The diff coverage is 100%.

@@              Coverage Diff               @@
##           scala_2.11    #3341      +/-   ##
==============================================
+ Coverage       93.46%   93.46%   +<.01%     
==============================================
  Files             385      385              
  Lines            7162     7166       +4     
  Branches          197      196       -1     
==============================================
+ Hits             6694     6698       +4     
  Misses            468      468

Impacted Files	Coverage Δ
core/src/main/scala/cats/syntax/alternative.scala	100% <100%> (ø)	⬆️

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