feat(tvix/eval): impl DoubleEndedIter for OwnedAttrsIterator

Change-Id: I4bd85dbe9c27047f4abbdeff4e2b796e9bcab3a1
Reviewed-on: https://cl.tvl.fyi/c/depot/+/10211
Reviewed-by: tazjin <tazjin@tvl.su>
Autosubmit: Adam Joseph <adam@westernsemico.com>
Tested-by: BuildkiteCI

tvix/eval/src/value/attrs.rs

@@ -615,3 +615,13 @@ impl ExactSizeIterator for OwnedAttrsIterator {
         }
     }
 }
+
+impl DoubleEndedIterator for OwnedAttrsIterator {
+    fn next_back(&mut self) -> Option<Self::Item> {
+        match &mut self.0 {
+            IntoIterRepr::Empty => None,
+            IntoIterRepr::Finite(inner) => inner.next_back(),
+            IntoIterRepr::Im(inner) => inner.next_back(),
+        }
+    }
+}