make RingMap an instance of Traversable

- some examples tested out by hand, but not thorough test case or even
  QuickCheck coverage

src/Hash2Pub/RingMap.hs

@@ -47,6 +47,13 @@ instance (Bounded k, Ord k) => Foldable (RingMap k) where
           traversingFL acc (ProxyEntry _ Nothing) = acc
           traversingFL acc (ProxyEntry _ (Just entry)) = traversingFL acc entry
 
+instance (Bounded k, Ord k) => Traversable (RingMap k) where
+    traverse f = fmap RingMap . traverse traversingF . getRingMap
+      where
+          traversingF (KeyEntry entry) = KeyEntry <$> f entry
+          traversingF (ProxyEntry to Nothing) = pure $ ProxyEntry to Nothing
+          traversingF (ProxyEntry to (Just entry)) = ProxyEntry to . Just <$> traversingF entry
+
 
 -- | entry of a 'RingMap' that holds a value and can also
 -- wrap around the lookup direction at the edges of the name space.