Function allocator_binder

3rd/asio-1.24.0/include/asio/bind_allocator.hpp:250–255

   */
  template <typename U>
  allocator_binder(const allocator_type& s,
      ASIO_MOVE_ARG(U) u)
    : allocator_(s),
      target_(ASIO_MOVE_CAST(U)(u))
  {
  }

  /// Copy constructor.
  allocator_binder(const allocator_binder& other)