Function async_answer2

examples/hello_coro.cpp:34–37

template <sender S1, sender S2>
auto async_answer2(S1 s1, S2 s2) -> stdexec::task<std::optional<int>>
{
  co_return co_await stopped_as_optional(async_answer(s1, s2));
}

// tasks have an associated stop token
auto async_stop_token() -> stdexec::task<std::optional<stdexec::inplace_stop_token>>