test VecDeque debug printing

tests/run-pass/vecdeque.rs

@@ -9,6 +9,14 @@ fn main() {
     let mut src = VecDeque::new();
     src.push_front(Box::new(2));
     dst.append(&mut src);
+    for a in dst.iter() {
+      assert_eq!(**a, 2);
+    }
+
+    // Regression test for Debug and Diaplay impl's
+    println!("{:?} {:?}", dst, dst.iter());
+    println!("{:?}", VecDeque::<u32>::new().iter());
+
     for a in dst {
       assert_eq!(*a, 2);
     }

tests/run-pass/vecdeque.stdout

@@ -0,0 +1,2 @@
+[2, 2] Iter([2, 2], [])
+Iter([], [])