make_unique should be an associated function

[adetaylor]

Right now, for a type called (e.g.) Bob, we generate a function called fn Bob_make_unique() -> UniquePtr<Bob>. It would be nicer if we can make that an associated function on the Bob type. This probably requires dtolnay/cxx#464 (or, of course, we do dtolnay/cxx#280 to add constructor support directly into cxx and remove it from autocxx).