scope.c: Don’t stringify globs on scope exit

scope.c: Don’t stringify globs on scope exit

This is a waste:

	    /* Can clear pad variable in place? */
	    if (SvREFCNT(sv) <= 1 && !SvOBJECT(sv)) {
		/*
		 * if a my variable that was made readonly is going out of
		 * scope, we want to remove the readonlyness so that it can
		 * go out of scope quietly
		 */
		if (SvPADMY(sv) && !SvFAKE(sv))
		    SvREADONLY_off(sv);

		if (SvTHINKFIRST(sv))
		    sv_force_normal_flags(sv, SV_IMMEDIATE_UNREF);

We can simply drop the globness in sv_force_normal instead of flatten-
ing globs to strings.  The same applies to COWs.  The SV_COW_DROP_PV
flag accomplishes both.

Before and after:

$ time ./miniperl -e 'for (1..1000000) { my $x = *foo }'

real	0m2.324s
user	0m2.316s
sys	0m0.006s
$ time ./miniperl -e 'for (1..1000000) { my $x = *foo }'

real	0m0.848s
user	0m0.840s
sys	0m0.005s