Specialized hashmap for u64 keys
Might be missing some functionality but you can remove, add, get and clear for now.
Performace compared to the standard hashmap:
test tests::u64_get_built_in ... bench: 43,925 ns/iter (+/- 2,712)
test tests::u64_get_intmap ... bench: 1,099 ns/iter (+/- 136)
test tests::u64_insert_built_in ... bench: 64,717 ns/iter (+/- 2,830)
test tests::u64_insert_intmap ... bench: 28,419 ns/iter (+/- 983)