Function pipeline

example/cpp14/executors/pipeline.cpp:159–174

// Launch the last stage in a pipeline.
template <class T, class F>
std::future<void> pipeline(queue_back<T> in, F f)
{
  // Get the function's associated executor, defaulting to thread_executor.
  auto ex = get_associated_executor(f, thread_executor());

  // Run the function, and as we're the last stage return a future so that the
  // caller can wait for the pipeline to finish.
  std::packaged_task<void()> task(
      [in, f = std::move(f)]() mutable
      {
        f(in);
      });
  std::future<void> fut = task.get_future();
  boost::asio::require(ex, execution::blocking.never).execute(std::move(task));
  return fut;
}

// Launch an intermediate stage in a pipeline.
template <class T, class F, class... Tail>