Make future::JoinHandle::abort actually cancel task

[bkragl]

The implementation of abort in #87 as detaching the task does not seem right, because detached tasks may still continue to run. Consider the following test, which should not fail but currently does.

#[test]
fn join_handle_abort_bug() {
    check_dfs(
        || {
            let (sender, receiver) = futures::channel::oneshot::channel();
            let t = future::spawn({
                async move {
                    receiver.await.unwrap();
                    panic!("should not get here");
                }
            });
            t.abort();
            sender.send(()).unwrap();
            shuttle::thread::yield_now();
        },
        None,
    );
}

(The yield_now is needed because otherwise the main task would immediately finish, in which case also the execution finishes because there are no attached tasks left.)

We need a way to actually cancel a task, which drops its continuation and returns a JoinError indicating cancellation.

[jamesbornholt]

abort is funny because it's best-effort and so non-deterministic when the task should stop. The current implementation doesn't do that. Maybe just always canceling the task immediately is a better default, but it can miss executions, in the same way our wait_timeout implementations can miss executions.