Remove redundant slicing

Clippy emits:

 warning: redundant slicing of the whole range

As suggested use `key` directly instead of the slice.

Author: tcharding

src/hmac.rs

@@ -85,10 +85,10 @@ impl<T: HashTrait> HmacEngine<T> {
                 *b_o ^= *b_h;
             }
         } else {
-            for (b_i, b_h) in ipad.iter_mut().zip(&key[..]) {
+            for (b_i, b_h) in ipad.iter_mut().zip(key) {
                 *b_i ^= *b_h;
             }
-            for (b_o, b_h) in opad.iter_mut().zip(&key[..]) {
+            for (b_o, b_h) in opad.iter_mut().zip(key) {
                 *b_o ^= *b_h;
             }
         };