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Eliminate xpad_cop_seq from _xnvu union
PVNV's used to be used to hold pad names (like '$lex'), but aren't used
for this purpose any more. So eliminate the xpad_cop_seq part of the
union.
Since S_scan_subst() was using xnv_u.xpad_cop_seq.xlow to store a
temporary line count, add a new union member for that.
The main usage of this field on CPAN is to define
COP_SEQ_RANGE_LOW()-style macros, so if the module is still using
xpad_cop_seq for that purpose, it's already broken.