Switch to Data.Ord.Down

[treeowl]

We don't really want our own version. We should make the switch once haskell/core-libraries-committee#23 is accepted and the change is made.

[konsumlamm]

If we're waiting for haskell/core-libraries-committee#23, the question arises if we only want to use Data.Ord.Down for newer versions (for which these optimizations are inplemented). My preference is to use it anyway, since it's a lot cleaner imo (and people who really need the performance can use a newer GHC).

[treeowl]

It's a separate module, so no big deal to keep it around for a while and only compile it when necessary. We can toss it a couple years later. Of course, that's all assuming my proposal is accepted....