RISC0 version bump and various fixes

[capossele]

Few suggestions:

• bump risc0-zkvm to 0.18
• simplify code
• fix how the input is fed to the guest on the merkle examples

In general running the prover with CPU vs metal vs CUDA can be achieved by just enabling the feature e.g., -F metal.

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Name	Status	Preview	Comments	Updated (UTC)
zk-benchmarks	✅ Ready (Inspect)	Visit Preview	💬 Add feedback	Sep 24, 2023 1:27am

[calummoore]

Thanks for the PR @capossele 👍