message: also use BigInt::from
diff: @@ -1095,11 +1095,11 @@ fn test_iter_product_generic() {
  #[test]
  fn test_pow() {
  - let one = 1u32.to_bigint().unwrap();
  - let two = 2u32.to_bigint().unwrap();
  - let minus_two = -2i32.to_bigint().unwrap();
  - let four = 4u32.to_bigint().unwrap();
  - let eight = 8u32.to_bigint().unwrap();
  + let one = BigInt::from(1i32);
  + let two = BigInt::from(2i32);
  + let four = BigInt::from(4i32);
  + let eight = BigInt::from(8i32);
  + let minus_two = BigInt::from(-2i32);
  macro_rules! check {
  ($t:ty) => {
  assert_eq!(two.pow(0 as $t), one);
  