Efficient single inheritance

Efficient single inheritance via virtual structs and unification and nesting of structs and enums.
@CloudiDust, I agree that it's not intuitive, but consider this: Swift adopted our same distinction between enums and structs, and this time next year Swift will be the fifth-most-popular programming language in the world. I agree that `data` would be philosophically cleaner, but we'd still need to provide a way to opt into either enum-sizing or struct-sizing, and it may be a misguided effort if Swift sets a precedent that we can exploit.