Test the size of every struct defined here.

src/jingle_ft.rs

@@ -348,6 +348,15 @@ mod tests {
     use hashes::Algo;
     use base64;
 
+    #[test]
+    fn test_size() {
+        assert_size!(Range, 48);
+        assert_size!(File, 184);
+        assert_size!(Description, 184);
+        assert_size!(Checksum, 216);
+        assert_size!(Received, 32);
+    }
+
     #[test]
     fn test_description() {
         let elem: Element = r#"