Test the size of every struct defined here.

src/sasl.rs

@@ -215,6 +215,18 @@ mod tests {
     use try_from::TryFrom;
     use minidom::Element;
 
+    #[test]
+    fn test_size() {
+        assert_size!(Mechanism, 1);
+        assert_size!(Auth, 32);
+        assert_size!(Challenge, 24);
+        assert_size!(Response, 24);
+        assert_size!(Abort, 0);
+        assert_size!(Success, 24);
+        assert_size!(DefinedCondition, 1);
+        assert_size!(Failure, 32);
+    }
+
     #[test]
     fn test_simple() {
         let elem: Element = "<auth xmlns='urn:ietf:params:xml:ns:xmpp-sasl' mechanism='PLAIN'/>".parse().unwrap();