RFC: deref coercions

Add the following coercions:

* From `&T` to `&U` when `T: Deref<U>`.
* From `&mut T` to `&U` when `T: Deref<U>`.
* From `&mut T` to `&mut U` when `T: DerefMut<U>`

These coercions eliminate the need for "cross-borrowing" (things like `&**v`) and calls to `as_slice`.

[Rendered](https://github.com/aturon/rfcs/blob/deref-coercions/active/0000-deref-coercions.md)
Once associated types land, we'd make `T` an associated type, which would force it to be uniquely determined by the `Self` type.

That restriction is necessary for the coercion algorithm being proposed to work.