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?