py/obj: Add "full" and "empty" non-variable-length mp_obj_type_t.

This will always have the maximum/minimum size of a mp_obj_type_t
representation and can be used as a member in other structs.

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

py/objnamedtuple.h

@@ -29,7 +29,10 @@
 #include "py/objtuple.h"
 
 typedef struct _mp_obj_namedtuple_type_t {
-    mp_obj_type_t base;
+    // Must use the full-size version to avoid this being a variable sized member.
+    // This means that named tuples use slightly more RAM than necessary, but
+    // no worse than if we didn't have slots/split representation.
+    mp_obj_full_type_t base;
     size_t n_fields;
     qstr fields[];
 } mp_obj_namedtuple_type_t;