#### experimental::co_spawn (2 of 3 overloads)

Spawn a new thread of execution.

    template<
        typename ExecutionContext,
        typename F,
        typename CompletionToken,
        typename  = typename enable_if<      is_convertible<ExecutionContext&, execution_context&>::value>::type>
    auto co_spawn(
        ExecutionContext & ctx,
        F && f,
        CompletionToken && token);