refactor!: change the output to `ProofPlan::verifier_evaluate` to `Result<IndexMap<ColumnRef, S>, ProofError>`

[iajoiner]

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Rationale for this change

In order to make ProofPlans composable it is better if we can feed the result of ProofPlan::verifier_evaluate back into another call of the same method.

What changes are included in this PR?

Change the output to ProofPlan::verifier_evaluate to Result<IndexMap<ColumnRef, S>, ProofError>

Are these changes tested?

Existing tests do pass.