Use byte instead of i

The identifier `i` is predominantly used for indexing an array but we
are using it as a place holder for the iterated value of an array that
is then printed. The identifier `byte` is more descriptive.

Done in preparation for adding similar code to the `ecdh` module.

src/secret.rs

@@ -101,8 +101,8 @@ impl fmt::Debug for DisplaySecret {
 
 impl fmt::Display for DisplaySecret {
     fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
-        for i in &self.secret {
-            write!(f, "{:02x}", i)?;
+        for byte in &self.secret {
+            write!(f, "{:02x}", byte)?;
         }
         Ok(())
     }