Refactor MSM to reduce code duplication

[waamm]

Description

• Consolidated duplicate functions: The previously separate functions msm_u8, msm_u16, msm_32, and msm_u64 have been replaced with a single generic function msm_small_int.
• Edited the comment above msm_binary regarding its range.
• Reduced the window size in msm_serial (it doesn't seem to have performance implications).

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Before we can merge this PR, please make sure that all the following items have been
checked off. If any of the checklist items are not applicable, please leave them but
write a little note why.

• Targeted PR against correct branch (master)
• Linked to GitHub issue with discussion and accepted design OR have an explanation in the PR that describes this work.
• Wrote unit tests (not needed for small refactor)
• Updated relevant documentation in the code (not needed for small refactor)
• Added a relevant changelog entry to the Pending section in CHANGELOG.md (not needed for small refactor)
• Re-reviewed Files changed in the GitHub PR explorer

[burdges]

Why not call this one msm_u64? No point adding the word generic unless you're doing something strange.

[waamm]

I guess I'd prefer msm_small_int since this function is barely related to u64, but up to you?

[burdges]

I've no real opinion aside from generic seemed redundent.

[waamm]

Ok I've renamed it (and replaced the invocations inside msm_signed). In hindsight, I'm not entirely sure this PR is timely as the code feels quite WIP: when I try a recent arkworks commit in my codebase instead of version 0.5.0, some functions using MSMs run a bit faster while others are much slower...

[Pratyush]

Hm I’d be interested to learn more about the cases where you experienced a slowdown; do you have a link?

[Pratyush]

Hm what machine are you running on? I wonder if some change happened to the underlying field arithmetic

[waamm]

The CPU is an M4 Max.

[Pratyush]

Hm I am also on a M3 CPU

[waamm]

I’ll try on an M1 later today or tomorrow and report back.

In the meantime, maybe this PR should be approved or closed and this discussion continued elsewhere?

[waamm]

Similar results on an M1. (Again, this is for switching from 0.5.0 to your branch, where I had to modify std on this line to ark_std to get it to compile, executing env RAYON_NUM_THREADS=2 cargo bench --bench range_proof.)