`algorithms/mod`: Make mul_redc alloc-free

[github-actions]

On 2023-07-29 @prestwich wrote in aaa37d2 “Merge pull request #277 from DaniPopes/no_alloc”:

Make mul_redc alloc-free

mod add;
pub mod div;
mod gcd;
mod mul;
#[cfg(feature = "alloc")] // TODO: Make mul_redc alloc-free
mod mul_redc;
mod ops;
mod shift;

pub use self::{

From src/algorithms/mod.rs:10