Allow all set entry types to be implicitly cast to their key/value type to allow for easier iteration

src/types.cpp

@@ -823,8 +823,7 @@ gb_internal bool type_ptr_set_exists(PtrSet<Type *> *s, Type *t) {
 
 	// TODO(bill, 2019-10-05): This is very slow and it's probably a lot
 	// faster to cache types correctly
-	for (auto const &entry : *s) {
-		Type *f = entry.ptr;
+	for (Type *f : *s) {
 		if (are_types_identical(t, f)) {
 			ptr_set_add(s, t);
 			return true;