It's mostly exploiting 2 things:

1) The bounds checks can be skipped, because "I know what I'm doing". I'm not 100% sure of this, but I think that when I do `probe = (probe + 1) & self.hash_mask, the compiler doesn't still know that the new probe is in bounds. Therefore, I'd be paying for the bounds check on every single iteration. I don't like that. Leaving easy optimizations on the table for a core data structure isn't my style. I'd rather it be "unsafe", but well used, tested, and fast, than have it be "safe" and well used and tested, but slightly slower. I guess I can turn them back on, but it just feels so wrong..

2) The second invariant is that an element in the array is valid `if hashes[i] != 0`. I don't think there's any way to tell the compiler this, so I think I'm doomed to unsafe code here, unless I want to waste words on useless options.