Hashmap get unchecked

[folkertdev]

This

• combines the hashmap fields into their own struct (clearer, and useful for testing)
• uses unchecked access into the hashmap (the index is a u16, and the length of the array is usize::MAX or larger)

This marginally improves performance for compression level 9, uses fewer instructions, but uses more unsafe. We'll need a better benchmarking setup to confidently say that this tradeoff is worth it.

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