#[deriving(Deref)] for newtype structs

[lilyball]

The deriving machinery should support #[deriving(Deref)] on newtype structs. Originally newtype structs could always be deref'd to return the wrapped type, and it's useful to be able to opt back into that behavior on a per-struct basis.

Deriving Deref should also derive DerefMut.

[steveklabnik]

Closing in favor of rust-lang/rfcs#261