Add obvious Outputable Integer instance.

Reviewers: bgamari

Reviewed By: bgamari

Subscribers: rwbarton, thomie

Differential Revision: https://phabricator.haskell.org/D4232

compiler/utils/Outputable.hs

@@ -788,6 +788,9 @@ instance Outputable Int64 where
 instance Outputable Int where
     ppr n = int n
 
+instance Outputable Integer where
+    ppr n = integer n
+
 instance Outputable Word16 where
     ppr n = integer $ fromIntegral n