Avoid bounds checking in loop

Slicing `src` checks bounds only once instead of on every iteration of loop.

src/gf2.rs

@@ -1,6 +1,7 @@
 pub fn add_assign_binary(dest: &mut [u64], src: &[u64]) {
-    for i in 0..dest.len() {
+    let len = dest.len();
+    for (dest, &src) in dest.iter_mut().zip(&src[..len]) {
         // Addition over GF(2) is defined as XOR
-        dest[i] ^= src[i];
+        *dest ^= src;
     }
 }