Test the size of every struct defined here.

src/ibr.rs

@@ -96,6 +96,11 @@ mod tests {
     use super::*;
     use compare_elements::NamespaceAwareCompare;
 
+    #[test]
+    fn test_size() {
+        assert_size!(Query, 152);
+    }
+
     #[test]
     fn test_simple() {
         let elem: Element = "<query xmlns='jabber:iq:register'/>".parse().unwrap();