Add declarations for hll_cnt_init and hll_cnt_free
Reimplement memory functions for hll_counter

Reimplement:

  - hll_cnt_new by means of hll_cnt_init
  - hll_cnt_delete by means of hll_cnt_free

To reduce code duplication.
Add function hll_cnt_copy_to
Implment hll_cnt_copy by means of hll_cnt_copy_to

To reduce code duplication