algorithms/div/small: Rewrite in a way that avoids bounds-checks without unsafe.

[github-actions]

On 2022-10-24 @recmo wrote in fb3aa20 “Merge pull request #172 from recmo/div2”:

Rewrite in a way that avoids bounds-checks without unsafe.

/// [MG10]: https://gmplib.org/~tege/division-paper.pdf
///
/// # Panics
///
/// May panics if the above requirements are not met.
// TODO: Rewrite in a way that avoids bounds-checks without unsafe.
#[inline(always)]
pub fn div_nx1(limbs: &mut [u64], divisor: u64) -> u64 {
    debug_assert!(divisor != 0);
    debug_assert!(!limbs.is_empty());
    debug_assert!(*limbs.last().unwrap() != 0);

From src/algorithms/div/small.rs:50