[perl #123223] Make PADNAME a separate type

[perl #123223] Make PADNAME a separate type

distinct from SV.  This should fix the CPAN modules that were failing
when the PadnameLVALUE flag was added, because it shared the same
bit as SVs_OBJECT and pad names were going through code paths not
designed to handle pad names.

Unfortunately, it will probably break other CPAN modules, but I think
this change is for the better, as it makes both pad names and SVs sim-
pler and makes pad names take less memory.