Add instances to ZipList
There was a reasonably uncontroversial vote on the libraries list to add common derived instances (Eq, Ord, Show, Read) to ZipList, which currently only has Functor and Applicative.
I have made the change on my local ghc build and it seems to build, but I can't get validate to pass. I'm not sure if it's my own fault, but considering I only made a one-line change, it seems odd. I'll attach a patch once I make sure it wasn't my fault.