Allow use of pre-hashed keys

As an optimisation, I want to be able to memoize a key-hash. My current solution is double-hashing with the first hash being key -> u64, then the second hash using a no-op hasher from u64 -> u64. However, this requires storing the hash twice in the hashmap.

I'm ok with such methods being unsafe if necessary, but the hashbrown equivalents are completely safe https://docs.rs/hashbrown/latest/hashbrown/hash_map/struct.RawEntryBuilder.html#method.from_key_hashed_nocheck

No file or test is named. Start by reviewing the existing map entry APIs and the linked hashbrown RawEntryBuilder::from_key_hashed_nocheck behavior; define the pre-hashed-key contract and add focused tests showing that a memoized hash works without storing it twice.

Written by the indexing model from the issue text.