feature: Support uint64 keys without converting to string then back again

Hey folks!

I have a use-case where we already use xxh3 for hashing various things, including the key for a previous cache's key. We're using bigcache now, but it's a bit awkward doing strconv.FormatUint and then configuring a dummy Hasher to do strconv.ParseUint.

Is there a reason this is omitted?