Merge branch '\N{} handler refactoring ' into blead

This series of commits refactors a portion of the \N{} handling in
regcomp.c.  This was done in preparation for allowing m'\N{...}', that
is single quoted patterns containing named characters.  Currently this
is illegal.

The final step to do this is not quite ready to be committed.  It
basically works, but the edge cases need to be looked at and tests
created, and the code freeze is upon us.

Some of the commits in this series are more than a year old, and I was
plagued with problems with rebasing, some of which I only resolved just
now, and decided to get this refactoring in before any more things come
along.  I intend to fix
https://rt.perl.org/Ticket/Display.html?id=132163
during the code freeze, and I've started it out on the refactored code
used here, so it would be extra work to have to use the unrefactored
version.