mp_identity(): Add generic identity function.

Useful as getiter method for objects which are their own iterators, etc.

py/obj.c

@@ -288,3 +288,9 @@ mp_obj_t mp_obj_len_maybe(mp_obj_t o_in) {
     }
     return MP_OBJ_NEW_SMALL_INT(len);
 }
+
+// Return input argument. Useful as .getiter for objects which are
+// their own iterators, etc.
+mp_obj_t mp_identity(mp_obj_t self) {
+    return self;
+}