Derived Functor instance for void types

Currently, if you write data V a deriving Functor GHC generates fmap _ _ = error "Void fmap" This seems quite unfortunate, because it loses potentially useful error information: fmap (+ 3) (error "Too many snozzcumbers!") throws "Void fmap", rather than the much more precise "Too many snozzcumbers!" I've opened Trac #13117 to fix this, but I figured I should double check that no one is opposed. David Feuer