Allow enum of integers (and any other type)

Closes #9826.

lib/types.nix

@@ -230,8 +230,15 @@ rec {
         substSubModules = m: submodule m;
       };
 
-    enum = values: mkOptionType {
+    enum = values:
-      name = "one of ${concatStringsSep ", " values}";
+      let
+        show = v:
+               if builtins.isString v then ''"${v}"''
+          else if builtins.isInt v then builtins.toString v
+          else ''<${builtins.typeOf v}>'';
+      in
+      mkOptionType {
+        name = "one of ${concatMapStringsSep ", " show values}";
         check = flip elem values;
         merge = mergeOneOption;
       };