RISC-V: instruction cache returns references

• closes RV-219

What

Return Option<&InstrCacheable> from the instruction cache, instead of Option<InstrCacheable>.

Why

This avoids copying the instruction bytes unnecessarily, resulting in a performance gain.

How

RISC-V: split instruction-cache from runnable m... (!15005 - merged) separated the instruction cache out from the rest of the 'runnable state' (ie parts of the state required to run cacheable instructions).

As a result, we can now safely return references from the instruction cache without rustc complaining.

Manually Testing

make -C src/riscv all

Benchmarking

	master	This MR	Improvement
AMD Ryzen 7 7840U	0.370 TPS (1/810 native)	0.440 TPS (1/682 native)	+18.9%
Benchmark Machine	0.189 TPS (1/799 native)	0.221 TPS (1/683 native)	+16.9%

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