NFC: Address a Clippy lint in limbs_mul.

This isn't caught by `mk/clippy.sh` usually since this code isn't used
for x86-64 targets yet.

src/arithmetic/montgomery.rs

@@ -178,12 +178,7 @@ fn limbs_mul(r: &mut [Limb], a: &[Limb], b: &[Limb]) {
     r[..ab_len].fill(0);
     for (i, &b_limb) in b.iter().enumerate() {
         r[ab_len + i] = unsafe {
-            limbs_mul_add_limb(
-                (&mut r[i..][..ab_len]).as_mut_ptr(),
-                a.as_ptr(),
-                b_limb,
-                ab_len,
-            )
+            limbs_mul_add_limb(r[i..][..ab_len].as_mut_ptr(), a.as_ptr(), b_limb, ab_len)
         };
     }
 }