Rename &mut to &only

The main guarantee provided by `&mut` is one of non-aliasability, but the name implies that the main guarantee is related to mutability. Additionally, `&mut` implies that `&` is immutable, which it isn't due to types such as `Cell` and `Mutex` that implement interior mutability. So, `&mut` is a misleading name. Rename it to `&only` which more closely reflects what it guarantees and also doesn't imply anything incorrect about `&`.