feat!: return PartialWitnessGeneratorStatus from PartialWitnessGenerator.solve

[TomAFrench]

Related issue(s)

Resolves #212

Description

Summary of changes

This PR makes the state of the PartialWitnessGenerator explicit in its return value rather than requiring callers to be aware of the internal logic by returning an enum describing the action which the caller must take.

Dependency additions / changes

(If applicable.)

Test additions / changes

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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

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