WIP: proof of concept patch for fixing quantified regex backrefs

Attached is as far as I've gotten with fixing depesz's complaint about
backrefs embedded within larger quantified expressions (the complaint
being that only the last match of the backref is checked properly).
This is per the analysis I posted at
https://sourceforge.net/tracker/index.php?func=detail&aid=1115587&group_id=10894&atid=110894
to the effect that the engine really has to have an "iteration" subre
type.

The patch is incomplete because I have not written the code yet for the
shortest-match-first case, only longest-match-first. Within that
restriction, it seems to work, though I have not yet tried the Tcl
regression tests on it.

I have to set this aside now and go focus on release tasks (like writing
release notes), so it's not going to be done in time to include in the
upcoming back-branch releases. I have mixed feelings about whether to
treat it as a back-patchable bug fix when it's done ... it's certainly a
bug fix but the odds of introducing new issues seem higher than average.
So maybe it should only go into HEAD anyway. Thoughts?

regards, tom lane