RISC-V PVM: Define PVM input type in Rust

Closes RV-205

What

This MR implements an equivalent of the PVM Sc_rollup.input type on the Rust side of the API.

Why

Previously, only set_input used this type on the OCaml side of the PVM, destructing it before passing the relevant arguments to different backend functions. With the addition of proofs, more functions will use it, making it easier to pass values of these types directly to the backend.

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.