o Added ability to set the ratio items:buckets.
o One can remove items while iterating.
o _htbl_tov now allocates the result array, _htbl_fillv acts as the old
  _htbl_tov did.

library support some day...

Use the fact that we only can store structs in the table to make the
complex pointer use safe. Unfortunately the equal and hash functions
need to take void * arguments.