Make ‘sub foo{}’ respect ‘our foo’
This commit switches all sub definitions, whether with ‘our’ or not,
to using S_force_ident_maybe_lex (formerly known as S_pending_ident).
This means that an unqualified (no our/my/state or package prefix)
‘sub foo’ declaration does a pad lookup, just like $foo.
It turns out that the vivification that I added to the then
S_pending_ident for CVs was unnecessary and actually buggy. We
*don’t* want to autovivify GVs for CVs, because they might be con-
stants or forward declarations, which are stored in a simpler form.
I also had to change the subname rule used by MYSUB in perly.y, since
it can now be fed a PRIVATEREF, which it does not expect. This may
prove to be temporary, but it keeps current tests passing.