Test the size of every struct defined here.

src/blocking.rs

@@ -98,6 +98,14 @@ generate_empty_element!(
 mod tests {
     use super::*;
 
+    #[test]
+    fn test_size() {
+        assert_size!(BlocklistRequest, 0);
+        assert_size!(BlocklistResult, 24);
+        assert_size!(Block, 24);
+        assert_size!(Unblock, 24);
+    }
+
     #[test]
     fn test_simple() {
         let elem: Element = "<blocklist xmlns='urn:xmpp:blocking'/>".parse().unwrap();