Implement Actor for async function without result

[Thomasdezeeuw]

Currently example 1b has the following actor.

async fn greeter_actor(_: Context<!>) -> Result<(), !> {
    println!("Hello World");
    Ok(())
}

I would like to following to be possible

async fn greeter_actor(_: Context<!>)  { // Returns `()`.
    println!("Hello World");
}

The following implementation doesn't work as it overlaps with our implementation for Future<Output = Result(), E>.

impl<Fut> Actor for Fut
    where Fut: Future<Output = ()>
{
    type Error = !;

    fn try_poll(self: Pin<&mut Self>, waker: &LocalWaker) -> Poll<Result<(), Self::Error>> {
        self.poll(waker).map(Ok)
    }
}

This requires rust-lang/rfcs#1672.

[Thomasdezeeuw]

Closing this for now, returning a result is fine.

[Thomasdezeeuw]

This is now implemented in #372.