Implement the 64-bit variant of xxHash.

Matches C in speed, but it looks like SipHash was not the only bottleneck for #11783. 

Tests include the official data and the relevant ones from SipHash.
It's a large prime. This should be `#[cfg(test)]` I think, but I wanted to be consistent with `SipHash` which has `new_with_keys(0, 0)` here.

For real uses, the seeds should be randomized, which is done with `RandomSipHasher` in `libstd`.