Test the size of every struct defined here.

src/mood.rs

@@ -272,6 +272,12 @@ mod tests {
     use try_from::TryFrom;
     use minidom::Element;
 
+    #[test]
+    fn test_size() {
+        assert_size!(MoodEnum, 1);
+        assert_size!(Text, 24);
+    }
+
     #[test]
     fn test_simple() {
         let elem: Element = "<happy xmlns='http://jabber.org/protocol/mood'/>".parse().unwrap();