A robin-hood hashing based hash table.

This is significantly faster than the old hashtable implementation, and
uses more than 20% less memory, with less cache pressure, too.

comprehensive_old_hashmap               ... bench:       378 ns/iter
comprehensive_new_hashmap               ... bench:       206 ns/iter
1.8x faster

old_hashmap_as_queue                    ... bench:       238 ns/iter
new_hashmap_as_queue                    ... bench:       119 ns/iter
2x faster

old_hashmap_insert                      ... bench:       172 ns/iter
new_hashmap_insert                      ... bench:       146 ns/iter
1.17x faster

old_hashmap_find_existing               ... bench:        50 ns/iter
new_hashmap_find_existing               ... bench:        35 ns/iter
1.43x faster

old_hashmap_find_notexisting            ... bench:        49 ns/iter
new_hashmap_find_notexisting            ... bench:        34 ns/iter
1.44x faster