Ability to pass ownership of unsized types

[nikomatsakis]

Currently you can't use unsized types as parameters or return values. There are good reasons to support this, including passing closures to fns and being able to write things like Rc::new(*slice) where slice has type Box<[T]> or what have you.

Postponed RFCs:

• Add unsized return values. #977

[Centril]

cc #1909 and rust-lang/rust#48055