message: Add convenience function
diff: @@ -158,6 +158,14 @@ public:
  dec_total_task_size(policy_.task_size(x));
  }
  + /// @private
  + task_size_type next_task_size() const noexcept {
  + if (head_.next == nullptr)
  + return 0;
  + auto ptr = promote(head_.next);
  + return policy_.task_size(*ptr);
  + }
  +
  /// @private
  unique_pointer next(task_size_type& deficit) noexcept {
  unique_pointer result;
  