Merge dissect() into cdissect() to remove a pile of near-duplicate code.

The "uncomplicated" case isn't materially less complicated than the full
case, certainly not enough so to justify duplicating nearly 500 lines
of code.  The only extra work being done in the full path is zaptreesubs,
which is very cheap compared to everything else being done here, and
besides that I'm less than convinced that it's not needed in some cases
even without backrefs.