feat(nargo): add nargo execute command

[TomAFrench]

Related issue(s)

Resolves #626

Description

Summary of changes

This PR adds the nargo execute command which is responsible for loading prover inputs and solving the witness for the circuit. This is essentially the Rust equivalent of solveAcirPartialWitness from the noir.js hackmd.

Benefits being:

• Better logical separation between the 3 steps of witness construction, proof generation and proof verification.
• Users can print out the return value of a circuit without having to go through with generating a proof and opening Verifier.toml.

I've named this nargo execute as I initially thought of it as a command to "execute the noir program and print its return value" but it may be worth renaming to something more "witness-y" let's leave it as execute as it's more understandable to non-ZK devs.

Dependency additions / changes

N/A

Test additions / changes

N/A

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

[TomAFrench]

@jfecher I'd be interested to get your input on this. Executing shaves off a significant amount of time relative to proving so this could be useful in speeding up #728.

[jfecher]

This looks good to me from my perspective. I'd like another approval before merging though.

In relation to #728, I think it would be faster for that PR but I also think for that PR I can change it to stop right after SSA or ACIR without proving at all since all failing constraints should be caught during SSA as everything would be constant due to there being no inputs.

[TomAFrench]

Cheers, this should probably wait until #731 is in at least.