Efficient single inheritance

Efficient single inheritance via virtual structs and unification and nesting of structs and enums.
I'd even be OK with these:
1. forall A, B: B has exactly A's first N fields  => Coerce<Fat<U, A>, Fat<U, B>>
2. forall A, B, U: Coerce<A, B> => Coerce<Fat<U, A>, Fat<U, B>>
3. Any DST with a vtable (fat pointer or Fat<U>) should be able to support downcasting (to an option).

I doubt think "forall A, B: B has exactly A's first N fields  => Coerce<A, B>" is safe, because the polymorphic instances (like my 2nd one, Coerce<A, B> => Coerce<&A, &B>, etc) rely on A and B having the same size.