support 32-bit hashes

[RaduBerinde]

The memory used to accumulate the hashes, as well as the memory used by NewBinaryFuse can be an issue. In Pebble (CockroachDB), we use 32-bit hashes to save on memory when building bloom filters (approach inherited from RocksDB).

Would it be acceptable to modify the library to accept both uint32 and uint64 keys? We could define a type Key { ~uint32 | ~uint64 }. I can work on a patch, but wanted to check if it would be accepted before I start.

[lemire]

@RaduBerinde Absolutely.

[RaduBerinde]

I'm finding this is not so easy if we want to use 32-bit hashes internally as well (and if we don't, the benefits of just supporting 32-bit input keys are not that great).