RISC-V: Introduce backend manager that allows state types to own their storage

Closes RV-196

What

Introduces a backend Manager that allows state data types that bind regions to own their backing storage.

Why

The owned regions aren't used yet. Building out changes that will be raised through subsequent MRs where I amend things around the state backend infrastructure, I found having an owning backend around makes things easier - particularly around tests.

Manually Testing

make -C src/riscv all

Benchmarking

No performance impact has been observed on the reference machine.

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 int™¡roduced 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.