chore(ssa refactor): Add entry point for acir gen pass

[jfecher]

Related issue(s)

Resolves #

Description

Summary of changes

Adds an entry point for the (not yet started) acir-gen pass for the new SSA IR. The entry point is Ssa::into_acir, and it is called within the new entry point function optitmize_into_acir which converts Program -> Ssa -> (optimizations) -> Acir.

Dependency additions / changes

Test additions / changes

Checklist

• I have tested the changes locally.
• I have formatted the changes with Prettier and/or cargo fmt with default settings.
• I have linked this PR to the issue(s) that it resolves.
• I have reviewed the changes on GitHub, line by line.
• I have ensured all changes are covered in the description.

Documentation needs

• This PR requires documentation updates when merged.

Additional context