Test the size of every struct defined here.

src/nick.rs

@@ -16,6 +16,11 @@ mod tests {
     use minidom::Element;
     use error::Error;
 
+    #[test]
+    fn test_size() {
+        assert_size!(Nick, 24);
+    }
+
     #[test]
     fn test_simple() {
         let elem: Element = "<nick xmlns='http://jabber.org/protocol/nick'>Link Mauve</nick>".parse().unwrap();