Efficient single inheritance

Efficient single inheritance via virtual structs and unification and nesting of structs and enums.
@CloudiDust, @dobkeratops: https://github.com/rust-lang/rfcs/pull/9 and https://github.com/rust-lang/rfcs/pull/91 allow for controlling where the vptr is (fat pointer or fat object) and for (among other things) safely casting between pointers to structs and pointers to their first member.

They would enable implementing C++ single inheritance class layout in a library, and even implementing different layouts for compatibility with different C++ compilers (if you run into one that changes) or different languages/manual class systems (COM, etc).