Test the size of every struct defined here.

src/bookmarks.rs

@@ -74,6 +74,13 @@ mod tests {
     use minidom::Element;
     use compare_elements::NamespaceAwareCompare;
 
+    #[test]
+    fn test_size() {
+        assert_size!(Conference, 152);
+        assert_size!(Url, 48);
+        assert_size!(Storage, 48);
+    }
+
     #[test]
     fn empty() {
         let elem: Element = "<storage xmlns='storage:bookmarks'/>".parse().unwrap();