Fix encoder for data type with phantom types

[Dmitrii Kovanikov]

Since it's not possible to get information about type variables from Generic instances, we need to write Elm instances for types with phantom type variables manually like this:

newtype Id a = Id { unId :: String }

instance Elm (Id a) where
    toElmDefinition _ = DefType $ ElmType "Id" ["a"] $ ElmConstructor "Id" [RefPrim ElmString] :| []

Unfortunately, because of that, encoder for Id is not what we want:

ghci> T.putStrLn $ prettyShowEncoder $ toElmDefinition $ Proxy @(Id Int)

encodeId : Ida -> Value
encodeId x = E.object <| case x of
    Id x1 -> [(tag, "Id"), ("contents", E.list [E.string x1])]

Though, when we use Id, everything is great!

λ: data User = User { userId :: Id User } deriving (Generic) deriving anyclass (Elm)
λ: T.putStrLn $ prettyShowDefinition $ toElmDefinition $ Proxy @User
type alias User =
    { id : Id User
    }

λ: T.putStrLn $ prettyShowEncoder $ toElmDefinition $ Proxy @User
encodeUser : User -> Value
encodeUser x = E.object
    [ ("id", encodeId User x.id)
    ]