RISC-V: Make in-memory backend comparable

Part of RV-149

What

Makes InMemoryBackend comparable.

Why

This allows us to rely less on methods that access the underlying byte-vector such as to_bytes which were previously used to compare backends.

Manually Testing

make -C src/riscv all

Benchmarking

Does not impact performance.

Tasks for the Author

• All Linear issues that relate to this MR were linked using magic words (e.g. part of, relates to, closes)
• There is no dead code and other spurious artefacts in my changes
• All newly introduced public functions, methods and types are documented
• Documentation of changed functions, methods and types is up-to-date
• There are tests which cover bugs which have been fixed, if any
• Reasonable effort has been made towards making CI pass
• Performance has been benchmarked and populated in the table above
• All the above was completed before assigning this MR to reviewers