Give back references to keys

[g2p]

FrozenMap hands out references to its values, but since the map also owns the keys it is possible to provide references to the keys as well.

Assuming K: StableDeref, implement:

fn insert_key_value(self, k: K, v: V) -> (&K::Target, &V::Target);
fn get_key_value(…) -> (&K::Target, &V::Target);

[Manishearth]

Ought to be fine as long as we appropriately guard things

[g2p]

There's a PR for get_key_value at #35.

I couldn't do insert_key_value, getting a key with the map lifetime from hash_map::Entry can't be done in one go / without cloning, see rust-lang/rust#65225 (comment) “On the topic of key ownership…” for a discussion on what needs to be done before something like that could be stabilized.

OccupiedEntry needs to keep a non-Optional reference to the key within the map.