The disposition concept describes types that can be used to test whether
an asynchronous operation completed without error. This includes
error_code and exception_ptr, but can be extended to user types via
specialisation of the disposition_traits class template.
Generic code that is intended to work in terms of dispositions should
not use asio::disposition_traits directly, but instead:
* Test whether a disposition holds no error by comparing against
asio::no_error (of type asio::no_error_t).
* Use asio::to_exception_ptr to convert a disposition to a
std::exception_ptr.
* Use asio::throw_exception to throw an exception that corresponds to
the disposition, after first testing the disposition against
asio::no_error.
The asio::use_future completion token and asio::awaitable<>-based
coroutines have been updated to work generically in terms of
dispositions.
