conversions between These and Disjunction

Added support for the exclusive-or case with conversions to and from `\/`.
@tonymorris Monocle depends upon scalaz and even provides lenses for some data structures: https://github.com/julien-truffaut/Monocle/blob/master/core/src/main/scala/monocle/std/Maybe.scala

I'm sure adding more supported structures would be welcome.