avoid murmur mixplit when keys are already hashes

[RaduBerinde]

In many cases the keys are already the result of a good hashing function like murmur or xxhash. It would make sense to avoid the murmur64 mixsplit on each key during building (and on each lookup).

I was thinking we could make it so that filter.Seed=0 means skipping the mixsplit step altogether. We could try that for the first iteration during building; if it doesn't work, we retry with a non-zero seed (and no sorting+removing of duplicates - we can do that if we fail again).

We could make this an option when using the builder.

[lemire]

It is reasonable. Pull request invited.

[RaduBerinde]

I will first profile some benchmarks to see if this would save anything significant.