Change visibility and minor refactors

[zkfriendly]

As part of updating WHIR version on Provekit and integrating with Buffer Abstraction, 2 minimal refactors were done:

1. Making Messages struct fields pub instead of pub(crate)
2. Moving concat from MathBuffer to BufferOps

A minor refactor on NTT tests was also done.

[codspeed-hq]

Merging this PR will not alter performance

✅ 10 untouched benchmarks
⏩ 22 skipped benchmarks¹

---

_{Comparing zkfr/integrate-buffer-abstraction (e0aec15) with main (92652ca)}

Footnotes

1. 22 benchmarks were skipped, so the baseline results were used instead. If they were deleted from the codebase, click here and archive them to remove them from the performance reports. ↩