RISC-V: Consolidate proof generation

What

Consolidates the code for proof generation.

Why

The NodePvm (used in the rollup node) and PvmStepper (used in the sandbox) each had their own proof generation method. To avoid deviation, we consolidate both into one method on the Pvm type.

Manually Testing

make -C src/riscv all

Benchmarking

No runtime performance impact has been measured.

Tasks for the Author

• Link all Linear issues related to this MR using magic words (e.g. part of, relates to, closes).
• Eliminate dead code and other spurious artefacts introduced in your changes.
• Document new public functions, methods and types.
• Make sure the documentation for updated functions, methods, and types is correct.
• Add tests for bugs that have been fixed.
• Put in reasonable effort to ensure that CI will pass.
  • make -C src/riscv
  • dune build src/lib_riscv
  • dune build src/rust_deps
• Benchmark performance and populate the table above if needed.
• Explain changes to regression test captures when applicable.
• Write commit messages to reflect the changes they're about.
• Self-review your changes to ensure they are high-quality.
• Complete all of the above before assigning this MR to reviewers.