Add needed JSON instances for PrivateState

src/Hanafuda/Message.hs

@@ -150,6 +150,10 @@ data PrivateState = PrivateState {
     , deck :: [Hanafuda.Card]
   } deriving Generic
 
+instance FromJSON PrivateState
+instance ToJSON PrivateState where
+  toEncoding = genericToEncoding defaultOptions
+
 data PublicPlayer = PublicPlayer {
       meld :: Hanafuda.Pack
     , yakus :: KoiKoi.Score