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/objpolyiter.c

@@ -48,10 +48,9 @@ STATIC mp_obj_t polymorph_it_iternext(mp_obj_t self_in) {
 MP_DEFINE_CONST_OBJ_TYPE(
     mp_type_polymorph_iter,
     MP_QSTR_iterator,
-    MP_TYPE_FLAG_NONE,
+    MP_TYPE_FLAG_ITER_IS_ITERNEXT,
     MP_TYPE_NULL_MAKE_NEW,
-    getiter, mp_identity_getiter,
-    iternext, polymorph_it_iternext
+    iter, polymorph_it_iternext
     );
 
 #if MICROPY_ENABLE_FINALISER
@@ -81,10 +80,9 @@ STATIC MP_DEFINE_CONST_DICT(mp_obj_polymorph_iter_locals_dict, mp_obj_polymorph_
 MP_DEFINE_CONST_OBJ_TYPE(
     mp_type_polymorph_iter_with_finaliser,
     MP_QSTR_iterator,
-    MP_TYPE_FLAG_NONE,
+    MP_TYPE_FLAG_ITER_IS_ITERNEXT,
     MP_TYPE_NULL_MAKE_NEW,
-    getiter, mp_identity_getiter,
-    iternext, polymorph_it_iternext,
+    iter, polymorph_it_iternext,
     locals_dict, &mp_obj_polymorph_iter_locals_dict
     );
 #endif