Fix the general case of quantified regex back-references.

 Fix the general case of quantified regex back-references.
 
 Cases where a back-reference is part of a larger subexpression that
 is quantified have never worked in Spencer's regex engine, because
 he used a compile-time transformation that neglected the need to
 check the back-reference match in iterations before the last one.
 (That was okay for capturing parens, and we still do it if the
 regex has *only* capturing parens ... but it's not okay for backrefs.)
 
 To make this work properly, we have to add an "iteration" node type
 to the regex engine's vocabulary of sub-regex nodes.  Since this is a
 moderately large change with a fair risk of introducing new bugs of its
 own, apply to HEAD only, even though it's a fix for a longstanding bug.