Test the size of every struct defined here.

src/receipts.rs

@@ -33,6 +33,12 @@ mod tests {
     use minidom::Element;
     use ns;
 
+    #[test]
+    fn test_size() {
+        assert_size!(Request, 0);
+        assert_size!(Received, 24);
+    }
+
     #[test]
     fn test_simple() {
         let elem: Element = "<request xmlns='urn:xmpp:receipts'/>".parse().unwrap();