[perl #29070] Add vstring set-magic

[perl #29070] Add vstring set-magic

Some operators, like pp_complement, assign their argument to TARG
(which copies vstring magic), modify it in place, and then call set-
magic.  That’s supposed to work, but vstring magic was remaining as it
was, such that ~v7 would still be treated as "v7" by vstring-aware
code, even though the resulting string is not "\7".

This commit adds vstring set-magic that checks to see whether the pv
still matches the vstring.  It cannot simply free the vstring magic,
as that would prevent $x=v0 from working.