The awaitable<>, co_spawn(), this_coro, detached, and redirect_error
facilities have been moved from the asio::experimental namespace to
namespace asio. As part of this change, the this_coro::token() awaitable
has been superseded by the asio::use_awaitable completion token.
Please note that the use_awaitable and redirect_error completion tokens
work only with asynchronous operations that use the new form of
async_result with member function initiate(). Furthermore, when using
use_awaitable, please be aware that the asynchronous operation is not
initiated until co_await is applied to the awaitable<>.
