Test the size of every struct defined here.

src/hashes.rs

@@ -133,6 +133,12 @@ mod tests {
     use try_from::TryFrom;
     use minidom::Element;
 
+    #[test]
+    fn test_size() {
+        assert_size!(Algo, 32);
+        assert_size!(Hash, 56);
+    }
+
     #[test]
     fn test_simple() {
         let elem: Element = "<hash xmlns='urn:xmpp:hashes:2' algo='sha-256'>2XarmwTlNxDAMkvymloX3S5+VbylNrJt/l5QyPa+YoU=</hash>".parse().unwrap();