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/objstr.c

@@ -2151,7 +2151,7 @@ MP_DEFINE_CONST_OBJ_TYPE(
     print, str_print,
     binary_op, mp_obj_str_binary_op,
     subscr, bytes_subscr,
-    getiter, mp_obj_new_str_iterator,
+    iter, mp_obj_new_str_iterator,
     buffer, mp_obj_str_get_buffer,
     locals_dict, &mp_obj_str_locals_dict
     );
@@ -2166,7 +2166,7 @@ MP_DEFINE_CONST_OBJ_TYPE(
     print, str_print,
     binary_op, mp_obj_str_binary_op,
     subscr, bytes_subscr,
-    getiter, mp_obj_new_bytes_iterator,
+    iter, mp_obj_new_bytes_iterator,
     buffer, mp_obj_str_get_buffer,
     locals_dict, &mp_obj_bytes_locals_dict
     );