Add qr/\p{Name=...}/

Add qr/\p{Name=...}/

This accomplishes the same thing as \N{...}, but only for regex
patterns, using loose matching and only the official Unicode names.

This commit includes a comparison of the two approaches, added to
perlunicode.  But the real reason to do this is as a way station to
being able to specify wild card lookup on the name property, coming in a
later commit.

I chose to not include user-defined aliases nor :short character names
at this time.  I thought that there might be unforeseen consequences of
using them.  It's better to later relax a requirement than to try to
restrict it.