`modular`: Reduce larger bit-sizes to smaller ones.

*On 2022-06-03 @recmo wrote in [`2f6a5d6`](https://github.com/recmo/uint/commit/2f6a5d6b21bed46723e8815a63a1724187aa59b1) “Merge pull request #103 from recmo/mulmod”:*

Reduce larger bit-sizes to smaller ones.

```rust
/// ⚠️ Compute $\mod{\mathtt{self}}_{\mathtt{modulus}}$.
///
/// **Warning.** This function is not part of the stable API.
///
/// Returns zero if the modulus is zero.
// FEATURE: Reduce larger bit-sizes to smaller ones.
#[must_use]
pub fn reduce_mod(mut self, modulus: Self) -> Self {
if modulus == Self::ZERO {
return Self::ZERO;
}

```
*From [`src/modular.rs:14`](https://github.com/recmo/uint/blob/2f6a5d6b21bed46723e8815a63a1724187aa59b1/src/modular.rs#L14)*