Finish challenge 25 and update License

src/utils.rs

@@ -39,3 +39,9 @@ pub fn read_hex_lines(path: &str) -> Vec<Bytes> {
         .map(|line| Bytes::from_hex(&line.unwrap()))
         .collect()
 }
+
+pub fn xor(a: &[u8], b: &[u8]) -> Vec<u8> {
+    Iterator::zip(a.iter(), b.iter())
+        .map(|z| *(z.0) ^ *(z.1))
+        .collect()
+}