chore: Miscellaneous refactor of CircuitSimplifier

[TomAFrench]

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Description

Summary of changes

I noticed some code which doesn't do anything in CircuitSimplifier so I removed it. While I was here I updated simplify_arithmetic to use Expression methods so the logic reads closer to "plain english". The trivial case for simplify_quotient (where a == 0 || pred == 0 so we zero everything out) has also been moved up to the top of the if-else chain to get it out of the way sooner.

Dependency additions / changes

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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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[TomAFrench]

@guipublic @kevaundray Can I get a review on this so we can put this in 0.15.0?

[TomAFrench]

Bumping to 0.16.0