Newtype deriving

[Rendered](https://github.com/aochagavia/rfcs/blob/0c685fcf3e8231ef5a782cd49c4437a876b3aded/text/0000-newtype-deriving.md)

See also the discussion on https://github.com/rust-lang/rust/issues/19597

cc @aturon @huonw 
`Show` can be derived via `#[deriving(Show)]`