Get FunctorFilter on par with the other MTL typeclasses

[barambani]

This adds FunctorFilter to Kleisli and IndexedStateT as from the issue #2556 . As a side note I tried to reduce the requirements for the IndexedStateT instance but I couldn't come up with anything that need less than Applicative and FlatMap, so I kept the implementation in the issue that requires Monad (see below as an example)

def mapFilter[A, B](fa: IndexedStateT[F, SA, SB, A])(f: A => Option[B]): IndexedStateT[F, SA, SB, B] =
  IndexedStateT[F, SA, SB, B] { sa =>
    FF.mapFilter(fa.run(sa)){ case (sb, a) => f(a) map (sb -> _) }
  }

I'm happy to change it though if there's something less restrictive.
There is still some work to do about the comments at the bottom of the ticket that I'm still investigating. I need more details there.

Also I'm planning to open some follow up PRs to add instances of TraverseFilter as well to Kleisli and IndexedStateT, and to add both to other transformers where they are still missing. Is it something that's needed ? Happy to stop if they are not a priority.

[codecov-io]

Codecov Report

Merging #2594 into master will increase coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #2594      +/-   ##
==========================================
+ Coverage   95.16%   95.16%   +<.01%     
==========================================
  Files         361      361              
  Lines        6634     6641       +7     
  Branches      294      301       +7     
==========================================
+ Hits         6313     6320       +7     
  Misses        321      321

Impacted Files	Coverage Δ
core/src/main/scala/cats/data/Kleisli.scala	97.24% <100%> (+0.1%)	⬆️
core/src/main/scala/cats/data/IndexedStateT.scala	89.89% <100%> (+0.31%)	⬆️

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[barambani]

As from #2556 (comment) the PR is ready for review.

[Baccata]

Sorry for the misleading comment, it looks like EitherT needs one too 😅

[barambani]

Thanks @Baccata , very true. There are some other that are still missing it and there might be also some that need a TraverseFilter instance. At the moment it is only provided for Option, OptionT and few other. As I commented above I would address them in a separate PR, but I want to make sure first that there is intention to have them.

[barambani]

@kailuowang , @LukaJCB do you have any bandwidth to have a look at this PR ? In case let me know if you'd like for me to add anything here. Thanks a lot.

[kailuowang]

sorry for the late response. I will take a look Monday

[barambani]

no worries. Many thanks

[kailuowang]

Thanks so much!

[barambani]

👍