Implement more robust

[yoshuawuyts]

When debugging tuple Merge with mw we found something which hints at a potential bug in tuple Merge. The test case was as follows:

let a = stream::once(async { () });
let b = stream::once(pending());
let c = (a, b).merge();
while let Some(_) = c.next().await {
    dbg!(item);
}
println!("done");

"done" should never be able to print in this case. When doing a cargo run it never triggered, but when running it through the debugger we could see it trigger. This may hint at a race condition!

[yoshuawuyts]

Okay the issue was we were using futures::pending! instead of futures::future::pending. It apparently exposed "poll once" semantics, and because we don't yet have "perfect wakers", it got polled twice - meaning it would resolve after the first future resolved.

No bug in the impl, just in the way we were testing it!