refactor: replace direct indexing for bus_prefix and beta_powers

[Nashtare]

Describe your changes

Match AIR trace access for clarity and safety upon updating / tweaking hardcoded constants.
LMK what you think @adr1anh, feel free to close if you think that's not appropriate

Checklist before requesting a review

• Repo forked and branch created from next according to naming convention.
• Commit messages and codestyle follow conventions.
• Commits are signed.
• Relevant issues are linked in the PR description.
• Tests added for new functionality.
• Documentation/comments updated according to changes.
• Updated `CHANGELOG.md'

[adr1anh]

I haven't taken a full look yet, but this is looking very nice!

It should play nicely into #2962. During the exploration of the tradeoff between trace generation and constraint evaluation, it became clear that we needed an extension point for groups of messages (e.g. Hasher and BlockHash). These groups have many mutually exclusive flags with ~7 possible messages, all sharing a very common structure. The naive constraint evaluation algorithm would encode each message individually, and some rough estimates showed this would double the number of EF operations.

The ideas being explored in the PR involve adding a hook for InteractionGroup which allows the caller to specialize the computation of the messages, caching common sub expressions. This requires direct access to the challenges, so this PR definitely helps with getting those correct.

[adr1anh]

After merging next into adr1anh/constraint-simplification, I'll cherry pick this into #2856

[adr1anh]

I opted not to include this in #2856 since the bus abstraction is getting its own PR. We should revisit this in the context of #2962.