Aggregate captures with _SAVE/_SAVE_AS

[xparq]

In "looping" rules (like ANY/MANY), or for SAVE_AS rules with the same name, repeated captures should append to a list, not just overwrite the last one...

• IOW, match collection should be transactional.

Only... this could wreak havoc with recursion; just trying to think about backtracking from partial matches hurts already.

Clearing the results on backtracking would be like stack unwinding with exception handling: there should be a reference baseline "catch" point in the backtracking path, where any tentatively successful "sub-results" should be unrolled on failure.

And that point is just there implicitly right before every match: if that match failed, everything that had been collected during that (i.e. by successful child (sub-) matches) should be undone, as if the match hadn't happened at all.

• Actually, with appends, just by remembering the last "watermark" positions in the results containers (e.g. vectors), unrolling on a failed match could work very efficiently.