Resolve needless_borrow clippy lint

    error: this expression borrows a value the compiler would automatically borrow
       --> tests/../src/lexical/math.rs:597:25
        |
    597 |         for (xi, yi) in (&mut x[xstart..]).iter_mut().zip(y.iter()) {
        |                         ^^^^^^^^^^^^^^^^^^ help: change this to: `x[xstart..]`
        |
        = note: `-D clippy::needless-borrow` implied by `-D clippy::all`
        = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#needless_borrow

src/lexical/math.rs

@@ -594,7 +594,7 @@ mod large {
 
         // Iteratively add elements from y to x.
         let mut carry = false;
-        for (xi, yi) in (&mut x[xstart..]).iter_mut().zip(y.iter()) {
+        for (xi, yi) in x[xstart..].iter_mut().zip(y.iter()) {
             // Only one op of the two can overflow, since we added at max
             // Limb::max_value() + Limb::max_value(). Add the previous carry,
             // and store the current carry for the next.