Figure out where repr would be useful

[hecatia-elegua]

If we want to allow this to replace num_enum for some tasks, it might be useful.
Besides that I'm not so sure.

• repr(packed), repr(align(n)) is not useful here, since we only have one field
• repr(transparent) should just work (would turn into the inner type)
• repr(uN) works on enums only, we could generate try/from impls for it
• repr(C) should be investigated more, we currently transmute nested array fields like this: [[T; N]; M] = [T; N*M] and some people say ordering could get messed up that way (though, the nesting usecase is quite rare)