py/obj: Merge getiter and iternext mp_obj_type_t slots.

The goal here is to remove a slot (making way to turn make_new into a slot)
as well as reduce code size by the ~40 references to mp_identity_getiter
and mp_stream_unbuffered_iter.

This introduces two new type flags:
- MP_TYPE_FLAG_ITER_IS_ITERNEXT: This means that the "iter" slot in the
  type is "iternext", and should use the identity getiter.
- MP_TYPE_FLAG_ITER_IS_CUSTOM: This means that the "iter" slot is a pointer
  to a mp_getiter_iternext_custom_t instance, which then defines both
  getiter and iternext.

And a third flag that is the OR of both, MP_TYPE_FLAG_ITER_IS_STREAM: This
means that the type should use the identity getiter, and
mp_stream_unbuffered_iter as iternext.

Finally, MP_TYPE_FLAG_ITER_IS_GETITER is defined as a no-op flag to give
the default case where "iter" is "getiter".

Signed-off-by: Jim Mussared <jim.mussared@gmail.com>

py/objlist.c

@@ -455,13 +455,13 @@ STATIC MP_DEFINE_CONST_DICT(list_locals_dict, list_locals_dict_table);
 MP_DEFINE_CONST_OBJ_TYPE(
     mp_type_list,
     MP_QSTR_list,
-    MP_TYPE_FLAG_NONE,
+    MP_TYPE_FLAG_ITER_IS_GETITER,
     list_make_new,
     print, list_print,
     unary_op, list_unary_op,
     binary_op, list_binary_op,
     subscr, list_subscr,
-    getiter, list_getiter,
+    iter, list_getiter,
     locals_dict, &list_locals_dict
     );