Function allocator_binder

include/boost/asio/bind_allocator.hpp:236–240

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

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