Laws for MonadFilter and MonadCombine

[non]

Both of these have laws that we aren't currently testing.

The important one is MonadFilter since it introduces a new method (empty) which isn't participating in any of its "inherited" laws. But there are also laws for how empty and combine interact with flatMap which we should be testing for MonadCombine as well.

See #125 for more discussion.

[xuwei-k]

cats.MonadFilter = MonadZero ?

https://wiki.haskell.org/MonadPlus_reform_proposal

[fthomas]

I'll work on this one.

[mpilquist]

@fthomas It might be good to do #212 first because MonadCombineLaws depend on AlternativeLaws.

[mpilquist]

Oh, and in order to write AlternativeLaws, you'll need the soon to be merged MonoidKLaws

[fthomas]

Thanks for the hint. I'll postpone the MonadCombineLaws then and just submit a PR for the MonadFilterLaws.

[fthomas]

This should be fixed by #198, #215, and #223.