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>

extmod/modbtree.c

@@ -319,15 +319,19 @@ STATIC const mp_rom_map_elem_t btree_locals_dict_table[] = {
 
 STATIC MP_DEFINE_CONST_DICT(btree_locals_dict, btree_locals_dict_table);
 
+STATIC const mp_getiter_iternext_custom_t btree_getiter_iternext = {
+    .getiter = btree_getiter,
+    .iternext = btree_iternext,
+};
+
 STATIC MP_DEFINE_CONST_OBJ_TYPE(
     btree_type,
     MP_QSTR_btree,
-    MP_TYPE_FLAG_NONE,
+    MP_TYPE_FLAG_ITER_IS_CUSTOM,
     MP_TYPE_NULL_MAKE_NEW,
     // Save on qstr's, reuse same as for module
     print, btree_print,
-    getiter, btree_getiter,
-    iternext, btree_iternext,
+    iter, &btree_getiter_iternext,
     binary_op, btree_binary_op,
     subscr, btree_subscr,
     locals_dict, &btree_locals_dict