Change every hll_cnt_* function to pointers.

Every function thet operates on HyperLogLog counters now receives and
returns pointers.
Implement hll_cnt_copy with memcpy.

Now hll_cnt_copy is implemented with calls to the function memcpy for
efficiency reasons.
Implement hll_cnt_equals with memcmp.

Now hll_cnt_equals is implemented by means of the memcmp function on the
register arrays, for efficiency reasons.