Call this function to signal to a stash that it has been assigned to
another spot in the stash hierarchy. C<stash> is the stash that has been
-assigned. C<oldstash> is the stash it replaces, if any. C<gv> is the glob
+assigned. C<oldstash> is the stash it replaces, if any. C<gv> is the glob
that is actually being assigned to.
This can also be called with a null first argument to