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 
While this is true, maybe you are only be interested in the wrapped value (e.g. showing `"2.0"` instead of `"Centimeters(2.0)"`). In that case it is useful to be able to derive `Show`.