Verify that type implements trait in vtable_for. Return None if not.

Author: ivarflakstad

library/coretests/tests/intrinsics.rs

@@ -198,6 +198,7 @@ fn carrying_mul_add_fallback_i128() {
 }
 
 #[test]
+#[allow(dead_code)]
 fn test_vtable_for() {
     #[derive(Debug)]
     struct A {}