Test the size of every struct defined here.

src/stream.rs

@@ -76,6 +76,11 @@ mod tests {
     use try_from::TryFrom;
     use minidom::Element;
 
+    #[test]
+    fn test_size() {
+        assert_size!(Stream, 216);
+    }
+
     #[test]
     fn test_simple() {
         let elem: Element = "<stream:stream xmlns='jabber:client' xmlns:stream='http://etherx.jabber.org/streams' xml:lang='en' version='1.0' id='abc' from='some-server.example'/>".parse().unwrap();