bug#32599: 25.2; Feature request: input PUA characters by name

[Eli Zaretskii <eliz@gnu.org>]

> From: jsbien@mimuw.edu.pl (Janusz S. Bień)
> Date: Sun, 26 May 2019 10:10:02 +0200
> 
> > First, the MUFI data in a more convenient form are available here:
> >
> > On Mon, Aug 27 2018 at  9:00 +0200, jsbien@mimuw.edu.pl writes:
> >
> > [...]
> >
> >> https://bitbucket.org/jsbien/unihistext/src/master/example/
> 
> If you prefer a file pattern after UnicodeData.txt, you can find it
> here:
> 
> http://www.kreativekorp.com/charset/PUADATA/PUBLIC/MUFI/
> 
> >
> > Secondly, other users may be interested in other sets of PUA characters,
> > cf.
> >
> > http://andron-typeforum.xobor.de/t10f13-Towards-a-linguistic-corporate-use-area-LINCUA.html
> > https://en.wikipedia.org/wiki/ConScript_Unicode_Registry
> 
> or Under-ConScript Unicode Registry:
> 
> http://www.kreativekorp.com/ucsur/

The UnicodeData.txt file is compiled into Emacs, but the files you
mention cannot be compiled into it, because they vary, and because
different users might want different lists of characters to be
supported.  So we need to design how this will work.

In addition, I think PUA codepoints aren't really treated as
characters in Emacs, so there's a need for some infrastructure
changes.

Patches welcome.