Rollup merge of #55485 - petertodd:2018-10-manuallydrop-deref, r=TimNN

Rollup merge of #55485 - petertodd:2018-10-manuallydrop-deref, r=TimNN

Return &T / &mut T in ManuallyDrop Deref(Mut) impl

Without this change the generated documentation looks like this:

    fn deref(&self) -> &<ManuallyDrop<T> as Deref>::Target

Returning the actual type directly makes the generated docs more clear:

    fn deref(&self) -> &T

Basically, compare how the impl for `Box<T>` and `ManuallyDrop<T>` looks in this screenshot:

![rust docs for ManuallyDrop as Deref](https://user-images.githubusercontent.com/7042/47673083-fc9dc280-db89-11e8-89b0-c6bde663feef.png)