fix slowdown in nested hash freeing
Commit
104d7b69 made sv_clear free hashes iteratively rather than recursively;
however, my code didn't record the current hash index when freeing a
nested hash, which made the code go quadratic when freeing a large hash
with inner hashes, e.g.:
my $r; $r->{$_} = { a => 1 } for 1..10_0000;
This was noticeable on such things as CPAN.pm being very slow to exit.
This commit fixes this by squirrelling away the old hash index in the
now-unused SvMAGIC field of the hash being freed.