message: Reduce allocations in shift benchmarks
diff: @@ -294,8 +294,9 @@ fn rand_131072(b: &mut Bencher) {
  #[bench]
  fn shl(b: &mut Bencher) {
  let n = BigUint::one() << 1000u32;
  - b.iter(|| {
  let mut m = n.clone();
  + b.iter(|| {
  + m.clone_from(&n);
  for i in 0..50 {
  m <<= i;
  }
  @@ -305,8 +306,9 @@ fn shl(b: &mut Bencher) {
  #[bench]
  fn shr(b: &mut Bencher) {
  let n = BigUint::one() << 2000u32;
  - b.iter(|| {
  let mut m = n.clone();
  + b.iter(|| {
  + m.clone_from(&n);
  for i in 0..50 {
  m >>= i;
  }
  