fix(acir): Fix Expression multiplication to correctly handle degree 1 terms

[TomAFrench]

Related issue(s)

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Resolves (link to issue)

Description

Summary of changes

There was a typo in the implementation of the Mul operator for Expressions.

acvm/acir/src/native_types/expression/operators.rs

Lines 196 to 205 in 45c45f7

	while i1 < self.linear_combinations.len() {
	let (a_c, a_w) = self.linear_combinations[i1];
	output.linear_combinations.push((rhs.q_c * a_c, a_w));
	i1 += 1;
	}
	while i2 < rhs.linear_combinations.len() {
	let (b_c, b_w) = rhs.linear_combinations[i1];
	output.linear_combinations.push((self.q_c * b_c, b_w));
	i2 += 1;
	}

Both of these while-loops use i1 to index into linear_combinations while the second should use i2. This results in either improper multiplication or panics when trying to read past the end of rhs.linear_combinations.

I've written a quick smoke test for the Mul operator along with another for Add (which was already covered by the smoketest for add_mul. The first commit shows this test catching the error and it is fixed in later commits.

I've also added an extra bit of logic to avoid zero-coefficient terms when multiplying Expressions together.

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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