Prepare proofs on benchmarks

[jmg-duarte]

To accurately calculate the weight of the proof verification, we need to be able to first create a valid proof.

In regular testing, this is not an issue as it uses std, however, when running benchmarks std is gone, as such we lose the ability to prepare a normal proof. We may be able to use fixed inputs and pre-calculate one, but this is, of course, not ideal.

The goal of this task is to prepare proofs as part of the benchmarks setup, be it fixed or on-demand.