Use Seq as base monad in the validator

It is more natural to implement the logic: try to apply each rule to
each node.

src/Language/GraphQL/AST/Encoder.hs

@@ -159,7 +159,7 @@ arguments :: Formatter -> [Argument] -> Lazy.Text
 arguments formatter = parensCommas formatter $ argument formatter
 
 argument :: Formatter -> Argument -> Lazy.Text
-argument formatter (Argument name value')
+argument formatter (Argument name value' _)
     = Lazy.Text.fromStrict name
     <> colon formatter
     <> value formatter value'