Backporting ifF for scala 2.11

[sdrafahl]

Backporting ifF for scala 2.11.

[travisbrown]

This unfortunately can't be added directly to the Functor trait on 2.11 because it breaks binary compatibility. You can add it to functor syntax via the BinCompat approach, and if you really wanted you could add it to Functor via an enrichment method.

[sdrafahl]

@travisbrown Could you show me an example of the BinCompat approach? I have an idea of how to do the implicit class approach but would like to consider both?

[travisbrown]

@sdrafahl This PR is a good example of what's necessary (it's quite painful, unfortunately, which is why we've dropped 2.11 on master 😄).

[codecov-io]

Codecov Report

Merging #3131 into scala_2.11 will increase coverage by 0.09%.
The diff coverage is 100%.

@@              Coverage Diff               @@
##           scala_2.11    #3131      +/-   ##
==============================================
+ Coverage        93.4%   93.49%   +0.09%     
==============================================
  Files             382      382              
  Lines            7155     7148       -7     
  Branches          195      190       -5     
==============================================
  Hits             6683     6683              
+ Misses            472      465       -7

Impacted Files	Coverage Δ
core/src/main/scala/cats/Functor.scala	100% <100%> (ø)	⬆️
core/src/main/scala/cats/data/NonEmptyList.scala	98.66% <0%> (-0.67%)	⬇️
core/src/main/scala/cats/data/NonEmptyVector.scala	99.13% <0%> (ø)	⬆️
core/src/main/scala/cats/instances/option.scala	100% <0%> (ø)	⬆️
.../src/main/scala/cats/laws/discipline/MiniInt.scala	97.29% <0%> (ø)	⬆️
core/src/main/scala/cats/instances/parallel.scala	0% <0%> (ø)	⬆️

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[larsrh]

I'm closing all 2.11-related tickets since there is not enough community interest in further releases.