RISC-V: Remove EnumCell

Closes RV-248

What

Removes EnumCell and replaces it with Cell.

Why

Cell is now powerful and performant enough that EnumCell is no longer needed. It also unlocks a tiny performance boost.

Manually Testing

make -C src/riscv all

Benchmarking

	target branch	This MR	Improvement
M3 MBP	2.509 TPS	2.624 TPS	4.58%
Benchmark Machine	2.048 TPS	2.072 TPS	1.18%

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.