doc: Add unit test for xor_into

rosenpass/src/util.rs

@@ -15,6 +15,17 @@ use std::{
 
 use crate::coloring::{Public, Secret};
 
+/// Xors a and b element-wise and writes the result into a.
+///
+/// # Examples
+///
+/// ```
+/// use rosenpass::util::xor_into;
+/// let mut a = String::from("hello").into_bytes();
+/// let b = b"world";
+/// xor_into(&mut a, b);
+/// assert_eq!(&a, b"\x1f\n\x1e\x00\x0b");
+/// ```
 #[inline]
 pub fn xor_into(a: &mut [u8], b: &[u8]) {
     assert!(a.len() == b.len());