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/Lexer.hs

@@ -100,8 +100,8 @@ amp :: Parser T.Text
 amp = symbol "&"
 
 -- | Parser for ":".
-colon :: Parser T.Text
-colon = symbol ":"
+colon :: Parser ()
+colon = symbol ":" >> pure ()
 
 -- | Parser for "=".
 equals :: Parser T.Text