make as_awaitable savvy to senders that are known to complete inline

[ericniebler]

awaiting a coroutine such as:

  auto burn_cycles() -> ex::task<int>
  {
    int result = 42;
    for (int i = 0; i < 1'000'000; ++i)
    {
      result += co_await ex::just(42);
    }
    co_return result;
  }

will no longer cause a stack overflow. for this optimization to kick in, the sender being co_await-ed must be known statically to complete inline always, but never complete with set_stopped.