Fix RefCell borrow_mut error on firing timeouts

The scope was a little longer than intended!

src/event_loop.rs

@@ -237,7 +237,11 @@ impl Loop {
     }
 
     fn consume_timeouts(&mut self, now: Instant) {
-        while let Some(idx) = self.timer_wheel.borrow_mut().poll(now) {
+        loop {
+            let idx = match self.timer_wheel.borrow_mut().poll(now) {
+                Some(idx) => idx,
+                None => break,
+            };
             trace!("firing timeout: {}", idx);
             let handle = self.timeouts.borrow_mut()[idx].1.fire();
             if let Some(handle) = handle {