Test the size of every struct defined here.

src/bind.rs

@@ -98,6 +98,11 @@ impl From<Bind> for Element {
 mod tests {
     use super::*;
 
+    #[test]
+    fn test_size() {
+        assert_size!(Bind, 80);
+    }
+
     #[test]
     fn test_simple() {
         let elem: Element = "<bind xmlns='urn:ietf:params:xml:ns:xmpp-bind'/>".parse().unwrap();