Override some methods in Kleisli instances

[peterneyens]

• Switch the order of the traits so that some methods are overridden by the implementation in other type class instances in its hierarchy

  For example:

  • extends Monad[Kleisli[F, A, ?]] with KleisliApplicative[F, A]
  • instead of extends KleisliApplicative[F, A] with Monad[Kleisli[F, A, ?]])

  We were already overriding ap in Monad, this also overrides product (which was the initial reason for this PR).
  For the rest we can just loose a couple of explicit overrides (eg super[KleisliSplit].split(f, g))

• Add a MonadReader test for Reader, which improves some code coverage.

[codecov-io]

Codecov Report

Merging #1618 into master will decrease coverage by 0.02%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #1618      +/-   ##
==========================================
- Coverage   93.11%   93.08%   -0.03%     
==========================================
  Files         250      250              
  Lines        3992     3979      -13     
  Branches      138      141       +3     
==========================================
- Hits         3717     3704      -13     
  Misses        275      275

Impacted Files	Coverage Δ
core/src/main/scala/cats/data/Kleisli.scala	92.64% <100%> (-1.19%)	⬇️

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[ceedubs]

Thanks @peterneyens!

This generally looks really good to me. One minor question: was there any particular motivation for changing a bunch of implicit parameter names to something like ev? I was thinking that in most other places in cats we were using implicit parameter names like F, M, AE etc, but admittedly I haven't looked around much to see if that's true.

[peterneyens]

Most of the parameters here in Kleisli were called ev, but it is probably best to be consistent in general, so I switched them around.

[kailuowang]

LGTM 👍