RISC-V PVM: OCaml API for proofs

Closes RV-205

What

This MR adds a mocked OCaml API for proofs and uses it in the OCaml side of the RISC-V PVM.

Why

Necessary step for integrating proof support.

How

Types and functions needed for proof support are mocked in the OCaml API, exposed through the backend and used in the OCaml side of the RISC-V PVM.

Manually Testing

make
dune exec ./tezt/tests/main.exe -- riscv

Benchmarking

This MR does not touch the RISC-V interpreter so performance is not impacted.

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.
• Benchmark performance and populate the table above if needed.
• 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.