make skip a double ended iterator

src/libcore/iter.rs

@@ -3851,6 +3851,17 @@ impl<I> Iterator for Skip<I> where I: Iterator {
 #[stable(feature = "rust1", since = "1.0.0")]
 impl<I> ExactSizeIterator for Skip<I> where I: ExactSizeIterator {}
 
+#[stable(feature = "double_ended_skip_iterator", since = "1.8.0")]
+impl<I> DoubleEndedIterator for Skip<I> where I: DoubleEndedIterator + ExactSizeIterator {
+    fn next_back(&mut self) -> Option<Self::Item> {
+        if self.len() > 0 {
+            self.iter.next_back()
+        } else {
+            None
+        }
+    }
+}
+
 /// An iterator that only iterates over the first `n` iterations of `iter`.
 ///
 /// This `struct` is created by the [`take()`] method on [`Iterator`]. See its