Shift to `using` directives outside the namespace

This is a concession from StyleCop defaults, but it's just too hard to paddle upstream with the C# language team, which doesn't consistently test their refactoring features when `using` is inside the namespace.

stylecop.json

@@ -10,6 +10,9 @@
       },
       "fileNamingConvention": "metadata",
       "xmlHeader": false
+    },
+    "orderingRules": {
+      "usingDirectivesPlacement": "outsideNamespace"
     }
   }
 }