RFC: matches and existsRight combinators

[mcanlas]

Includes two independent proposals per #614

1. matches partial pattern match combinator
2. existsRight combinator for asserting ApplicativeError

Proposal 2 is more a strawman, I'm not confident in it at all. I had to also demand Foldable. I looked at Bifoldable briefly but I wasn't sure how to merge the two channels into one. Felt like that was describing two fully independent channels vs a disjoint union in an ApplicativeError