#137 These-Disjunction Prism

This pull request includes the implementation of the These-Disjunction Prism, an example, property test, and a generator for These.
This[Int] extends 5 \&/ Nothing.
There aren't constructors for These as there are for Maybe.just and Maybe.nothing.
I'm sure some of the annotations are redundant, but since I had to use them at some places, I went and did it all around.