Resolve explicit_iter_loop pedantic clippy lint

    error: it is more concise to loop over references to containers instead of using explicit iteration methods
       --> tests/../src/lexical/math.rs:339:19
        |
    339 |         for xi in x.iter_mut() {
        |                   ^^^^^^^^^^^^ help: to write this more concisely, try: `&mut *x`
        |
        = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#explicit_iter_loop
        = note: `-D clippy::explicit-iter-loop` implied by `-D clippy::pedantic`

    error: it is more concise to loop over references to containers instead of using explicit iteration methods
       --> tests/../src/lexical/math.rs:485:19
        |
    485 |         for xi in x.iter_mut() {
        |                   ^^^^^^^^^^^^ help: to write this more concisely, try: `&mut *x`
        |
        = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#explicit_iter_loop

src/lexical/math.rs

@@ -336,7 +336,7 @@ mod small {
     pub fn imul(x: &mut Vec<Limb>, y: Limb) {
         // Multiply iteratively over all elements, adding the carry each time.
         let mut carry: Limb = 0;
-        for xi in x.iter_mut() {
+        for xi in &mut *x {
             carry = scalar::imul(xi, y, carry);
         }
 
@@ -482,7 +482,7 @@ mod small {
         let rshift = bits - n;
         let lshift = n;
         let mut prev: Limb = 0;
-        for xi in x.iter_mut() {
+        for xi in &mut *x {
             let tmp = *xi;
             *xi <<= lshift;
             *xi |= prev >> rshift;