objexcept: Optimize traceback allocation for exception.

Traceback allocation for exception will now never lead to recursive
MemoryError exception - if there's no memory for traceback, it simply
won't be created.

py/objlist.h

@@ -35,4 +35,6 @@ typedef struct _mp_obj_list_t {
     mp_obj_t *items;
 } mp_obj_list_t;
 
+mp_obj_t mp_obj_new_list_maybe(mp_uint_t n);
+
 #endif // __MICROPY_INCLUDED_PY_OBJLIST_H__