The buffer protocol type only has a single member, and this existing layout
creates problems for the upcoming split/slot-index mp_obj_type_t layout
optimisations.
If we need to make the buffer protocol more sophisticated in the future
either we can rely on the mp_obj_type_t optimisations to just add
additional slots to mp_obj_type_t or re-visit the buffer protocol then.
This change is a no-op in terms of generated code.
Signed-off-by: Jim Mussared <jim.mussared@gmail.com>