feat: add OccupiedEntry.get_entry_mut, VacantEntry.insert_entry

[jhgg]

Adds 2 utility functions ontop of the Entry API.

• OccupiedEntry.get_entry_mut - for when you want a reference to the key, and a mutable reference to the value.
• VacantEntry.insert_entry - for when you want to insert into the map, and return a reference to the key, and a mutable reference to the value.

[cuviper]

Has anything like this been proposed for the std maps? Ideally, we would keep a compatible API, and these methods don't seem particular to IndexMap.

[jhgg]

No :( I just needed these for a project I'm working on - and felt like it'd be cool to upstream it here. I don't have the time to get these into std tho.

[glaebhoerl]

A similar API was recently added to hashbrown: rust-lang/hashbrown#145