mk_partse_set: Avoid empty Parse_set elements

[ampli]

Significantly reduce the number of the Parse_set elements. It addresses the issue where mk_parse_set() encounters unconnected elements in the absence of a complete parse for a word range. This not only conserves memory but also reduces CPU usage.

In the same occasion:

• Refactor for readability.
• Improve the x_table stats (also needed to check the Parse_set saving).
• Add a comment on a bug if PERFORM_COUNT_HISTOGRAMMING is defined.

[linas]

This fails the python unit tests; I get

bindings/python-examples/lg_testutils.py'>
............................................s...............sFs.........E.............s.s..............link-grammar: Fatal error:
Assertion (pc != NULL) failed in list_links() (../../link-grammar/parse/extract-links.c:920): walked off the end in list_links

[ampli]

I have to remember to run the tests...
I force-pushed the fix, and also some minor changes.