Mix the hash's upper 16bits with the lower 16bits as a different hash for the
bloom filter. This means bloomfilter hits don't necessarily align with
occupied hashtable entries, possibly avoiding multiple hashtable probes to
detect a hashtable miss.

Add and use tmask attribute for getting the hashtable index from a hash.

Add and use bshift attribute for getting the bloomfilter bit index from a
hash. This means we use the upper bits of the hash instead of bit-mixing and
masking.

For a minimum of 1 and an empty file, the mask is empty, resulting in a bshift
with as many bits as unsigned hash. In C shifting by all the bits is
undefined.