A handler must satisfy the requirements of Destructible
(C++Std [destructible]) and MoveConstructible
A handler may expose additional information and behaviour via associators,
such as associated_allocator, associated_executor, and associated_cancellation_slot.
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)