Efficient single inheritance

Efficient single inheritance via virtual structs and unification and nesting of structs and enums.
@CloudiDust, for new users, we likely wouldn't even mention that you can nest structs in enums, and vice versa. From this point of view, there would be no unification. If you accept the notion that the vast majority of people will not need this sort of nesting (this is up for debate), then for practical purposes as well there will be no unification.

That said, I would welcome a proposal that would *actually* attempt to unify structs and enums under a single `data` keyword. But reusing the `unsized` keyword for that proposal would just be misleading, since neither structs nor enums are unsized in the DST sense.