`cmp`: Replace with `self == other` and deprecate once `PartialEq` is

*On 2026-08-31 @decofe wrote in [`a9450b4`](https://github.com/alloy-rs/ruint/commit/a9450b499c03f259238c2955b9512de873a77e12) “chore: apply latest nightly rustfmt (#627)”:*

Replace with `self == other` and deprecate once `PartialEq` is
const.

```rust
pub const fn const_eq(&self, other: &Self) -> bool {
as_primitives!(self, other; {
u64(x, y) => return x == y,
u128(x, y) => return x == y,
});
// TODO: Replace with `self == other` and deprecate once `PartialEq` is
// const.
let a = self.as_limbs();
let b = other.as_limbs();
let mut equal_count = 0usize;
const_range_for!(i in 0..LIMBS => {
equal_count += (a[i] == b[i]) as usize;

```
*From [`src/cmp.rs:165`](https://github.com/alloy-rs/ruint/blob/a9450b499c03f259238c2955b9512de873a77e12/src/cmp.rs#L165)*