Test the size of every struct defined here.

src/date.rs

@@ -43,6 +43,11 @@ mod tests {
     use chrono::{Datelike, Timelike};
     use std::error::Error as StdError;
 
+    #[test]
+    fn test_size() {
+        assert_size!(DateTime, 16);
+    }
+
     #[test]
     fn test_simple() {
         let date: DateTime = "2002-09-10T23:08:25Z".parse().unwrap();