Add missing specializations on DoubleEndedIterator impl

Resolves #65

src/lib.rs

@@ -940,6 +940,17 @@ where
     fn next_back(&mut self) -> Option<Self::Item> {
         for_both!(*self, ref mut inner => inner.next_back())
     }
+
+    fn nth_back(&mut self, n: usize) -> Option<Self::Item> {
+        for_both!(*self, ref mut inner => inner.nth_back(n))
+    }
+
+    fn rfold<Acc, G>(self, init: Acc, f: G) -> Acc
+    where
+        G: FnMut(Acc, Self::Item) -> Acc,
+    {
+        for_both!(self, inner => inner.rfold(init, f))
+    }
 }
 
 impl<L, R> ExactSizeIterator for Either<L, R>