Add builtin::export_lexically

[leonerd]

Adds a builtin:: function for performing the lexical-export behaviour, so that new code can do the same trickery that core builtin:: itself achieves.

[leonerd]

Thanks @ilmari. Comments addressed and force-pushed.

[ilmari]

LGTM

[tonycoz]

#19857 is still a problem here:

tony@venus:.../git/perl5$ ./perl -Ilib -Mbuiltin=lexically_export -e 'BEGIN { lexically_export abc => sub {}; } BEGIN { eval "abc()" }'
Built-in function 'builtin::lexically_export' is experimental at -e line 1.
perl: op.c:13404: Perl_find_lexical_cv: Assertion `PARENT_PAD_INDEX(name)' failed.
Aborted

[ilmari]

#19857 is still a problem here:

Seeing as that's a problem with lexical subs in general, not specifically with exporting them, I don't think that should block this.

[leonerd]

P5P requests that this be expanded into a full RFC. I am now in the process of writing that. Until that's agreed, we should not merge this (but it can be referenced as a prototype implementation)

[leonerd]

RFC now written and PR raised: Perl/PPCs#20