`modular`: Test for larger (>= m) values of a, b.

*On 2022-06-09 @recmo wrote in [`f0f0ddb`](https://github.com/recmo/uint/commit/f0f0ddb07d33f4bc350b1a18cf218dcebf64cf7a) “Merge pull request #126 from recmo/redc”:*

Test for larger (>= m) values of a, b.

```rust
let inv = (-inv).as_limbs()[0];

let r = U::from(2).pow_mod(U::from(64 * LIMBS), m);
let ar = a.mul_mod(r, m);
let br = b.mul_mod(r, m);
// TODO: Test for larger (>= m) values of a, b.

let expected = a.mul_mod(b, m).mul_mod(r, m);

assert_eq!(ar.mul_redc(br, m, inv), expected);
}

```
*From [`src/modular.rs:282`](https://github.com/recmo/uint/blob/f0f0ddb07d33f4bc350b1a18cf218dcebf64cf7a/src/modular.rs#L282)*