Test the size of every struct defined here.

src/idle.rs

@@ -27,6 +27,11 @@ mod tests {
     use std::str::FromStr;
     use std::error::Error as StdError;
 
+    #[test]
+    fn test_size() {
+        assert_size!(Idle, 16);
+    }
+
     #[test]
     fn test_simple() {
         let elem: Element = "<idle xmlns='urn:xmpp:idle:1' since='2017-05-21T20:19:55+01:00'/>".parse().unwrap();