Fix parallelize workload imbalance

[mratsim]

This fixes the parallel workload imbalance mentioned in my PSE/Taiko codebase review.

Currently if we take 40 items divided into 12 threads (AMD Ryzen 7800X, Apple M2 Pro or Intel i5-12600) the partitioning will lead to 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 7 = 3*11 + 7 = 40. The “remainder” thread will have 2.33x more work to do.

This can be quite extreme for example if we have 351 items to split on 32 cores, 351/32 = 10.96, rounded to 10 with integer division. 10*31 = 310, the last core needs to process 41 items, 4.1x more than the others.

This ensures whatever the number of items and the number of threads, the workload varies by at most 1.

This probably explains why not all cores are used while benchmarking (benchmarks TODO).

Unsafe

Note: I'm not familiar with Rust standard library and parallelism development since October 2016 (my last foray into writing Rust programs).
I've looked into:

• https://doc.rust-lang.org/std/primitive.slice.html
• Rayon chunking: https://github.com/rayon-rs/rayon/blob/2c97943/src/slice/chunks.rs

Note of the chunking routines provides balanced partitioning of a range.

• Either the last chunk can be extremely large like with the current usage of chunks_mut
• or it's not returned like in array_chunks and requires a separate remainder call
• or we get more chunks than our number of threads like with as_chunks

I might very well have missed something though.

[mratsim]

Upstreamed at privacy-scaling-explorations#186