PFDEBUG TODENSE added.

Converts HyperLogLogs from sparse to dense. Used for testing.
HyperLogLog invalid representation error code set to INVALIDOBJ.
More HyperLogLog tests.
ZLEXCOUNT implemented.

Like ZCOUNT for lexicographical ranges.
Pass by pointer and release of lex ranges.

Given that the code was written with a 2 years pause... something
strange happened in the middle. So there was no function to free a
lex range min/max objects, and in some places the range was passed by
value.
Basic ZRANGEBYLEX / ZLEXCOUNT tests.