.opposite() method implied on square direction.

[SylvanFranklin]

It's an almost trivial match statement to reverse a square direction, but I have found myself doing it enough times for the sake of logic, that It would be a very nice feature to have.

However this does feel a little bit necessary and like it could be handled by the end users, and also logically if the SquareDirection enum implied this it would also have to be implied in the other directional enums, and also it seems like it is kinda in she same vein as cardinal rotation.