Support serial composability in Validated

[metasim]

Per gitter comment, consider some mechanism for composing multiple, serialized Validated transformations without having to go back and forth between Xor to get flatMap.

[ceedubs]

To add a little more detail after a follow-up Gitter conversation. It sounds like the consensus is to add a new method to Validated that has the same signature as flatMap but with a different name, since it will not actually be a monadic bind (since it won't be consistent with ap).

As is often the case, naming is the hardest part. @metasim suggested andThen, which is perhaps a bit overloaded, but I've thought for a while and can't come up with a better name, so I'd be happy with that.