Instead of needing different switch cases for
converting countries to regions in multiple places,
we now only need a single country-to-region switch case
(in DiscIO/Enums.cpp), and we get a nice Region type.
At first there weren't many enums in Volume.h, but the number has been
growing, and I'm planning to add one more for regions. To not make
Volume.h too large, and to avoid needing to include Volume.h in code
that doesn't use volume objects, I'm moving the enums to a new file.
I'm also turning them into enum classes while I'm at it.
