Add right and left functor to BiFunctor

[LukaJCB]

Quick and dirty, should resolve #1834.

[codecov-io]

Codecov Report

Merging #1847 into master will increase coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #1847      +/-   ##
==========================================
+ Coverage   94.96%   94.97%   +<.01%     
==========================================
  Files         241      241              
  Lines        4173     4199      +26     
  Branches      106      111       +5     
==========================================
+ Hits         3963     3988      +25     
- Misses        210      211       +1

Impacted Files	Coverage Δ
core/src/main/scala/cats/functor/Bifunctor.scala	100% <100%> (ø)	⬆️
core/src/main/scala/cats/Monad.scala	96% <0%> (-0.16%)	⬇️
...ws/src/main/scala/cats/kernel/laws/OrderLaws.scala	100% <0%> (ø)	⬆️
core/src/main/scala/cats/syntax/monad.scala	100% <0%> (ø)	⬆️
core/src/main/scala/cats/syntax/cartesian.scala	50% <0%> (ø)	⬆️
free/src/main/scala/cats/free/Free.scala	91.04% <0%> (+1.68%)	⬆️

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[non]

Could we use a private abstract class (or even private case class) instead of trait?

Classes have a better binary compatibility story on 2.10 and 2.11 and I don't think we'll ever want to use these traits as mix-ins.

[LukaJCB]

Done :)

[non]

Thanks! LGTM 👍