Reinstate errors for unused variables and imports.

components/msg/lib.rs

@@ -2,6 +2,8 @@
  * License, v. 2.0. If a copy of the MPL was not distributed with this
  * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
 
+#![deny(unused_imports, unused_variable)]
+
 extern crate azure;
 extern crate geom;
 extern crate layers;