Add a method onto Rc and Arc to unwrap their inner content in a Box

[jjpe]

@InfernoDeity

No, because Rc::try_unwrap and Box::new exist only for T: Sized, even with the unsized_locals feature.

The Box API docs suggest differently, with a T: ?Sized bound on the struct definition, and no other bounds on the impl block or the Box::new() method.