Optimized HashMap. RawTable exposes a safe interface. (reopen)

This is #15720, rebased and reopened.

cc @nikomatsakis
Just to be sure I understand: This is super unlikely to to map to a non-zero bucket because the only set bit is super large, so when mod'd by the capacity it will only be non-0 if the capacity is super large, implying a hash table the size of basically the maximum theoretical 64-bit address space?