join should yield item immediately if either stream is ready

If one of the streams is ready and the other is pending, we should
just return the item from the ready stream. Otherwise, join will never
resolve until both streams are ready (which could be never).

Fixes #12.

src/join.rs

@@ -278,14 +278,14 @@ where
             // If one side is pending, we can't return Ready until that gets resolved.  Because we
             // have already requested that our child streams wake us when it is possible to make
             // any kind of progress, we meet the requirements to return Poll::Pending.
-            (PollState::Item(a, t), PollState::Pending) => {
-                *this.state = JoinState::A(a, t);
-                Poll::Pending
-            }
-            (PollState::Pending, PollState::Item(b, t)) => {
-                *this.state = JoinState::B(b, t);
-                Poll::Pending
-            }
+            (PollState::Item(a, t), PollState::Pending) => Poll::Ready(PollResult::Item {
+                data: a,
+                ordering: t,
+            }),
+            (PollState::Pending, PollState::Item(b, t)) => Poll::Ready(PollResult::Item {
+                data: b,
+                ordering: t,
+            }),
             (PollState::Pending, PollState::Pending) => Poll::Pending,
             (PollState::Pending, PollState::NoneBefore) => Poll::Pending,
             (PollState::NoneBefore, PollState::Pending) => Poll::Pending,