Add Signal to rtic-sync

[AdinAck]

Add the Signal structure to rtic-sync.

A Signal has unlimited writers and one reader, many writes can occur before a read, all overwriting the previous.

A reader can asynchronously wait for a write, requiring the write to occur during the lifetime of the wait future, or immediately resolve from previous writes (up to the user).

That's it, thanks!

[korken89]

Thanks for adding! I have some comments that need looking into.

[korken89]

Thanks for the updates! Can you have a look at adding tests?
You can copy the tests from channel if you need inspiration.

[AdinAck]

Sure! I also planned on adding an entry to examples later, I can include that in this PR along with the tests if you like.

[AdinAck]

Ok I think I added all the necessary tests.

I'm not quite sure about the soundness of the last test though, it works on my machine and the GitHub action, basically I yield the test thread to get tokio to poll the reader future waiting for a "fresh" write before the write occurs. What are your thoughts?

#[tokio::test]
async fn waiting() {
    static READER: StaticCell<SignalReader<u32>> = StaticCell::new();
    let (mut writer, reader) = make_signal!(u32);

    writer.write(0xaa);

    let reader = READER.init(reader);
    let handle = tokio::spawn(reader.wait_fresh());

    tokio::task::yield_now().await; // encourage tokio executor to poll reader future
    assert!(!handle.is_finished()); // verify reader future did not resolve after poll

    writer.write(0xab);

    assert!(handle.await.is_ok_and(|value| value == 0xab));
}