regex: Make \R simple again.

regex: Make \R simple again.

Commit 565fc1bb88638c2490cdab7a1055007f6b2d577c was based on my
misunderstanding of what regrepeat() operates on.  It is designed to
operate on nodes that are quantifiable without having to be
parenthesized.  So, in 'a?', the node generated for 'a' is quantifiable
as-is, but if you want to quantify 'ab', you have to parenthesize them,
like '(?:ab)?'

\R is such a node, since you can say things like '\R+'.  There was
indeed a bug, and that commit fixed it, but suboptimally.

This patch essentially reverts the commit mentioned above, and solves
the problem in a better way; it adds comments so that future
maintainters will be less likely to be led astray than I was.

It also adds a 'SIMPLE' flag for an expression that in my previous
misunderstanding didn't think was simple.  (It's enclosed in
parentheses, and hence can serve as an operand to a quantifier.)