hllSparseAdd() sanity check for span != 0 added.
hllSparseToDense API changed to take ref to object.

The new API takes directly the object doing everything needed to
turn it into a dense representation, including setting the new
representation as object->ptr.
PFDEBUG added, PFGETREG removed.

PFDEBUG will be the interface to do debugging tasks with a key
containing an HLL object.