Improve `instance Ixed (HashMap k a)` in `lens`

[sjakobi]

Can we expose the tooling around LookupRes from proper public modules, so we can improve the instance like this?

 type instance IxValue (HashMap k a) = a
 instance (Eq k, Hashable k) => Ixed (HashMap k a) where
-  ix k f m = case HashMap.lookup k m of
-     Just v  -> f v <&> \v' -> HashMap.insert k v' m
-     Nothing -> pure m
+  ix k f m = case HashMap.Internal.lookupRecordCollision h k m of
+     HashMap.Internal.Present v i ->
+       f v <&> \v' -> HashMap.Internal.insertKeyExists i h k v' m
+     HashMap.Internal.Absent -> pure m
+    where h = HashMap.Internal.hash k
   {-# INLINE ix #-}

For now, I suspect that even an implementation via ixAt (i.e. alterF) might be more efficient than the current one.

[treeowl]

We could certainly implement ix directly.