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;
+}

py/obj.h

@@ -351,6 +351,8 @@ extern const mp_obj_type_t fun_native_type;
 extern const mp_obj_type_t fun_bc_type;
 void mp_obj_fun_bc_get(mp_obj_t self_in, int *n_args, uint *n_state, const byte **code);
 
+mp_obj_t mp_identity(mp_obj_t self);
+
 // generator
 extern const mp_obj_type_t gen_instance_type;