[perl #101486] Make PL_curstash refcounted

[perl #101486] Make PL_curstash refcounted

This stops PL_curstash from pointing to a freed-and-reused scalar in
cases like ‘package Foo; BEGIN {*Foo:: = *Bar::}’.

In such cases, another BEGIN block, or any subroutine definition,
would cause a crash.  Now it just happily proceeds.  newATTRSUB and
newXS have been modified not to call mro_method_changed_in in such
cases, as it doesn’t make sense.