Proposal: Add missing Monoid for ZipList

[M Farkas-Dyck]

On 25/07/2015 at 15:50:40 -0500, Christopher Allen wrote:
> instance Monoid a => Monoid (ZipList a) where
>   mempty  = pure mempty
>   mappend = liftA2 mappend

+1

It's appropriately zippy.