Add API functions for regex global pos magic

[leonerd]

This PR adds a set of API functions for XS to see or modify any pos() magic position on an SV.

While reviewing some of my modules' more internals-hackery uses of the MAGIC API, I found a place where I have to reach deep into innards of how PERL_MAGIC_regexp_global works, in order to see or manipulate the pos() position annotation on a string SV. See this XS function:
https://metacpan.org/release/PEVANS/Term-TermKey-0.17/source/lib/Term/TermKey.xs#L713

This makes the code very tightly dependent on exact details of how Perl's magic currently works.

By adding these API functions (which can be easily emulated by earlier perls), the XS code can now be made much more robust against inner changes that may later affect how Perl implements this.

I'm not super convinced by the naming prefix on these - sv_regexp_global_pos_... might be considered rather too long. Will take suggestions on better ideas.

• This set of changes requires a perldelta entry, and I haven't yet written it. I'll do so before merge.

[leonerd]

Will also take ideas on whether some kind of flag should be provided to request operating by byte counts instead of character counts.

Ugh, actually as soon as I go to actually try out using this new API in my module mentioned above, I find that module specifically wants to operate in byte counts, rather than character counts.

I guess some kind of flag is warranted there then after all. I will think on this tomorrow.

[leonerd]

Now with (ab)use of SV_CATBYTES flag to request counting in bytes rather than chars.