RISC-V: dispatch instr functions during block construction

• closes RV-296
• closes RV-179

What

Dispatch Instruction::run_* during block construction.

Why

This avoids a match on every single instruction run, and instead use the pre-dispatched function over the state directly.

How

We leverage the EnrichedCell introduced in !15595 (merged) - we derive the function to run from the given opcode when we write the instruction into the block cache.

Manually Testing

make -C src/riscv all

Benchmarking

	!15595 (merged)	This MR	Improvement
AMD Ryzen 7 7840U	3.193 TPS	3.881 TPS	+21.5%
Benchmark Machine	2.452 TPS	2.937 TPS	+19.8%

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.