No 128-bit output

[rokicki]

We are considering using this to replace CityHash, where we use both CityHash64() and CityHash128() (in different contexts). Rapidhash only provides a 64-bit output value; it would be useful to also have a version that provided 128 bits of hash.

[Nicoshev]

@rokicki I plan to work on it, unfortunately measuring and tuning quality is a tedious labor.

For now, feel free to replace CityHash64 with rapidhash and continue to use CityHash128

[amadvance]

To get a 128-bit output, what about replacing the ending:

return rapid_mix(a ^ rapid_secret[7], b ^ rapid_secret[1] ^ i);

with something like this:

	a ^= rapid_secret[7];
	b ^= rapid_secret[1] ^ i;
	rapid_mum(&a, &b);
	rapid_write64(digest, a);
	rapid_write64(digest + 8, b);

Is that too simplistic? In any case, a proper 128-bit function would also need a 128-bit seed.