Relax ordering of transition

The first ordering of no operation relies on SeqCst semantics.
All works with Acquire/Release semantics, so there is no reason to use the SeqCst ordering.

The second ordering argument is for the failure, in which case we don't read any data,
and therefore don't need any synchronization.

src/lib.rs

@@ -404,7 +404,7 @@ impl<Rq, Rp> Channel<Rq, Rp> {
 
     fn transition(&self, from: State, to: State) -> bool {
         self.state
-            .compare_exchange(from as u8, to as u8, Ordering::SeqCst, Ordering::SeqCst)
+            .compare_exchange(from as u8, to as u8, Ordering::AcqRel, Ordering::Relaxed)
             .is_ok()
     }