feat: add OccupiedEntry.get_entry_mut, VacantEntry.insert_entry #118
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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.