add lens to library

Author: JBetz

patch.cabal

@@ -35,6 +35,7 @@ library
                , containers >= 0.6 && < 0.7
                , dependent-map >= 0.3 && < 0.4
                , dependent-sum >= 0.6 && < 0.7
+               , lens >= 4.7 && < 5
                , semigroupoids >= 4.0 && < 6
                , transformers >= 0.5.6.0 && < 0.6
                , witherable >= 0.3 && < 0.3.2