Add a insert_unique_hashed_nocheck method

[Zoxc]

This adds a insert_unique_hashed_nocheck method which inserts a key-value pair being given a hash for the key and not checking if the key already exists. It's a more powerful variant of insert_unique_unchecked since it takes a hash value and returns RawOccupiedEntryMut. I found a use for it in rustc's query system.

[Amanieu]

I would like to deprecate the raw entry API in favor of HashTable in #466. Could you have a look at that to see if it addresses your use case better?

[bors]

☔ The latest upstream changes (presumably #468) made this pull request unmergeable. Please resolve the merge conflicts.