Data.Either.Validation

This is the strawman PR for us to whack around, I don't expect this to be the final version.

The obvious possible improvements/additions would be:

Iso between Validation and Either. I did look at https://github.com/ekmett/lens/wiki/How-can-I-write-lenses-without-depending-on-lens%3F#iso but decided to just put this up now and experiment on my own time.

Polymorphic prisms for _Success/_Failure that work on Either and Validation like the Validate class in Tony's library.

No Monad instance for now. With AMP there shouldn't be any reason not to add the Monad instance back in with ap being more properly defined in terms of Applicative. That said, I think it'd be nicer if I could add the Monad instance as it would eliminate one possible reason to need to jump between Validation and Either.