try zeroing the subclasses first
zero the subclasses of ICLASSes as well
fix subclasses /included_in methods
gotta return ST_CONTINUE from here or everything gets shredded
we don't need to clear a method cache tree if we're gc'ing a class - its subclasses are unreachable now anyway
if we're sweeping an rb_method_entry_t, we've already cleared the cache
if we're looking at a module, zero the klass of the ICLASS
Merge remote-tracking branch 'jamesgolick/klasscache' into klasscache

Conflicts:
	gc.c
	version.h