Work-in-progress on attributes-v2

It's a little over two years since I first suggested the idea [1], and
a year since I wrote the PPC [2], but I've fiiiiinally got around to
actually writing some real code.

So far all I've really done is tidy up a bunch of internals. There's
now a new `attributes.c` file that is overall responsible for the
low-level mechanism, and maintains a table of known built-in attribute
names. Those built-in ones are the existing sub, class, or field
attributes. As it stands, I don't think there's much point merging it
yet as it's mostly building a base for the next stages.

My next plans are to carry on working on the ideas of the PPC doc;
namely, to create some kind of mechanism that out-of-core modules (i.e.
XS modules) can provide new lexically-scoped attribute definitions.
Since the entire point of that is to permit such additions from outside
of core, there won't really be much point using it in core. In order to
get a better feel for it and test it out, I am thinking I'll make
another "XS::Parse..." module, in similar theme to XS::Parse::Keyword
and XS::Parse::Infix.

Another part of the plan, which is mentioned briefly in the PPC doc as
a possible extension, is to think about how we can add attributes in
new locations, such as subroutine signature parameters. I think this
would be a good fit along with the other things I've been doing in
signatures, so there could be some good combinations there.

While I'm working on that, if people want to take a look at what I have
so far and give any comments ahead of an actual PR time, my
work-in-progress branch is sitting at:

  https://github.com/leonerd/perl5/tree/attributes-v2


[1]:
https://www.nntp.perl.org/group/perl.perl5.porters/2023/11/msg267303.html

[2]:
https://github.com/Perl/PPCs/blob/main/ppcs/ppc0029-attributes-v2.md

-- 
Paul "LeoNerd" Evans

[email protected]
http://www.leonerd.org.uk/  |  https://metacpan.org/author/PEVANS