RISC-V: Specialise impl Serialise/Deserialise for 1-element regions

What

Adds special serialisation and deserialisation cases for single-element regions.

Why

Representing 1-element regions as the element helps when serialising structures to inspect differences as this eliminates a level of indirection that would otherwise be distracting.

I used this when debugging something in !15006 (merged).

Manually Testing

make -C src/riscv all

Benchmarking

This doesn't impact performance.

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.