Test the size of every struct defined here.

src/stanza_id.rs

@@ -42,6 +42,12 @@ mod tests {
     use error::Error;
     use std::str::FromStr;
 
+    #[test]
+    fn test_size() {
+        assert_size!(StanzaId, 96);
+        assert_size!(OriginId, 24);
+    }
+
     #[test]
     fn test_simple() {
         let elem: Element = "<stanza-id xmlns='urn:xmpp:sid:0' id='coucou' by='coucou@coucou'/>".parse().unwrap();