objtype: Implement ->getiter() method for instances.

Includes support for native bases.

tests/basics/subclass_native_specmeth.py

@@ -0,0 +1,18 @@
+# Test calling non-special method inherited from native type
+
+class mylist(list):
+    pass
+
+l = mylist([1, 2, 3])
+print(l)
+print([e for e in l])
+
+
+class mylist2(list):
+
+    def __iter__(self):
+        return iter([10, 20, 30])
+
+l = mylist2([1, 2, 3])
+print(l)
+print([e for e in l])