Small cyclotomic-rings crate refactor

[v0-e]

Here we have,

• Renames cyclotomic-rings to latticefold-rings crate (crates/rings directory). cyclotomic-rings was a bit generic, while the crate itself is specialized for the repo;
• Renames the LatticefoldChallengeSet to ChallengeSet;
• Renames short to small challenges, matching the paper;
• In places where the CS generic was employed, now we reach for the SuitableRing::ChallengeSet when available.