Fix GNFData instance for V1 #19
Description
Currently, we have
instance GNFData V1 where grnf = error "Control.DeepSeq.rnf: uninhabited type"For old GHC, we should have
grnf x = x `pseq` error "Control.DeepSeq.rnf: uninhabited type"For newer GHC, we should have
grnf x = case x of {}