Fix hlint warnings for hlint v2.1.20 (#1320)

* Apply some hlint suggestions.

* Simplify config parser (to avoid hlint error)

* Reorganize for clarity (and fix hlint error)

* Remove redundant language extension

* Reformat slice more conventionally to avoid hlint bug

* Refactor for clarity (and to avoid hlint error)

* Simplify (and avoid hlint error)

* Fix hlint complaint

src/PostgREST/Error.hs

@@ -3,8 +3,7 @@ Module      : PostgREST.Error
 Description : PostgREST error HTTP responses
 -}
 {-# OPTIONS_GHC -fno-warn-orphans #-}
-{-# LANGUAGE FlexibleInstances    #-}
-{-# LANGUAGE TypeSynonymInstances #-}
+{-# LANGUAGE FlexibleInstances #-}
 
 module PostgREST.Error (
   errorResponseFor