Cleanup, document, and restructure regen/regcomp.pl

Cleanup, document, and restructure regen/regcomp.pl

We cleanup the parsing code, replacing our set of arrays of
properties with an array of hashes of properties, with
utility subs registering new items, etc.

We also split up the output code into a set of subs,
one sub per output "blob" (generaly a var definition),
so that we have some visibility of the higher level strucuture
of our output code. With this patch visibility of the structure
of what we generate emerges from the nest of here docs. :-)

Note this change does not (greatly) alter regcomp.sym or
perldebguts.pod, it merely cleans up and generally speaking
modernizes and most importantly documents the code.