Add `HashMap::get_key_value_mut`
Add `HashMap::get_key_value_mut`

This new API exposes a way to modify a value of an HashMap while keeping
a reference to the relative key. The other safe way of doing this is
using the `RawEntryMut` API, which could easily lead to an
inconsistent state of the map if misused.

rust-lang/hashbrown,rust-lang/hashbrown