RISC-V: bump rust-toolchain to 1.78.0; bump ocaml-rs

What

Follow up to Rust: Default to 1.78 and update `rust_deps` (!15024 - merged), we bump our rust version to match that used by rust_deps.

Why

It's good to ensure the versions we test with match was it actually used in the rollup node.

NB the bump in rust version introduced the warnings from the SDK, which we needed to silence. We use global imports there to avoid a large nest of feature flags w.r.t. the entrypoint.

Manually Testing

make -C src/riscv all

Benchmarking

	master	This MR	Improvement
AMD Ryzen 7 7840U	0.612 TPS (1/402 native)	0.625 TPS (1/396 native)	+2.1%
Benchmark Machine	0.277 TPS (1/611 native)	0.280 TPS (1/589 native)	+1.1%

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.

/assign @ole.kruger
/assign @victor-dumitrescu
/assign @felix.puscasu1
/assign @anastasia.courtney
/assign @emturner

/assign_reviewer @ole.kruger
/assign_reviewer @victor-dumitrescu
/assign_reviewer @felix.puscasu1
/assign_reviewer @anastasia.courtney
/assign_reviewer @emturner

/unassign me
/unassign_reviewer me

/ready