Convert regex engine's subre tree from binary to N-ary style.

 Convert regex engine's subre tree from binary to N-ary style.
 
 Instead of having left and right child links in subre structs,
 have a single child link plus a sibling link.  Multiple children
 of a tree node are now reached by chasing the sibling chain.
 
 The beneficiary of this is alternation tree nodes.  A regular
 expression with N (>1) branches is now represented by one alternation
 node with N children, rather than a tree that includes N alternation
 nodes as well as N children.  While the old representation didn't
 really cost anything extra at execution time, it was pretty horrid
 for compilation purposes, because each of the alternation nodes had
 its own NFA, which we were too stupid not to separately optimize.
 (To make matters worse, all of those NFAs described the entire
 alternation pattern, not just the portion of it that one might
 expect from the tree structure.)
 
 We continue to require concatenation nodes to have exactly two
 children.  This data structure is now prepared to support more,
 but the executor's logic would need some careful redesign, and
 it's not clear that a lot of benefit could be had.
 
 This is part of a patch series that in total reduces the regex engine's
 runtime by about a factor of four on a large corpus of real-world regexes.
 
 Patch by me, reviewed by Joel Jacobson
 
 Discussion: https://postgr.es/m/1340281.1613018383@sss.pgh.pa.us