librustc: Implement the syntax in the RFC for unboxed closure sugar.

Part of issue #16640. I am leaving this issue open to handle parsing of
higher-rank lifetimes in traits.

This change breaks code that used unboxed closures:

* Instead of `F:|&: int| -> int`, write `F:Fn(int) -> int`.

* Instead of `F:|&mut: int| -> int`, write `F:FnMut(int) -> int`.

* Instead of `F:|: int| -> int`, write `F:FnOnce(int) -> int`.

[breaking-change]