Construct an async result from a given handler.
Handler & );
When using a specalised async_result,
the constructor has an opportunity to initialise some state associated
with the handler, which is then returned from the initiating function.
Distributed under the Boost Software License, Version 1.0. (See accompanying
file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt)