conversions between These and Disjunction

Added support for the exclusive-or case with conversions to and from `\/`.
I think it's OK. It's got more than one sensible inhabitant though, so maybe some disambiguation would good.

I also think we could use monocle here, because there is quite a useful `Prism` that falls out of this operation. I am not sure how to go about integrating monocle with scalaz, but I do know that lens is a decent replacement for Predef. If I had my time over, almost all of scalaz would depend on monocle/some-lens-library.