Move VecDeque::retain to vec_deque_retain feature.

Author: Stebalien

src/libcollections/vec_deque.rs

@@ -1415,7 +1415,7 @@ impl<T> VecDeque<T> {
     /// let v: Vec<_> = buf.into_iter().collect();
     /// assert_eq!(&v[..], &[2, 4]);
     /// ```
-    #[unstable(feature = "collections",
+    #[unstable(feature = "vec_deque_retain",
                reason = "new API, waiting for dust to settle")]
     pub fn retain<F>(&mut self, mut f: F) where F: FnMut(&T) -> bool {
         let len = self.len();