feat(acvm)!: make Backend trait object safe

[TomAFrench]

Related issue(s)

Resolves #179

Description

Summary of changes

I've spun out PartialWitnessGenerator::any_missing_assignment and PartialWitnessGenerator::solve_directives into standalone functions and added a &self parameter to PartialWitnessGenerator::solve_black_box_function_call.

Dependency additions / changes

(If applicable.)

Test additions / changes

I've also added a test to ensure that the Backend trait remains object safe (I couldn't find any lint, etc. which would enforce this so I've just attempted to pass a trait object to a function as this will fail to compile should Backend not be object safe.

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.

Additional context

(If applicable.)