chore(acvm)!: Make internals of ACVM private

[TomAFrench]

Description

Problem*

Resolves #354

Summary*

This PR restricts the public interface of the ACVM so that we can make changes to the internal solvers without it being a breaking change.

Motivation for this PR being that we've removed the need for backends to implement 80% of the black box solvers so we don't need to expose them (at which point we might as well handle the rest of the interface). Once this is done then I can refactor these to expose the underlying hash function implementations out to ACVM simulator (#300) without having to worry about further breaking changes.

Filtering this change through to noir atm to test. Done

Additional Context

PR Checklist*

• I have tested the changes locally.
• I have formatted the changes with Prettier and/or cargo fmt on default settings.

[kevaundray]

Seems okay to flatten the signature structure and once we get more signatures we can add it back