Refactoring to improve type safety

[Dentosal]

Waiting for #805 and possibly other changes, as this is quite aspiritional.

Work towards and enabled by #746.

Use type safe RegId instead of RegisterId which is just usize. Get rid of WriteRegKey.

This PR tries really hard to uphold full test and branch coverage while removing lots of test code. I'm hoping to split off some of the work to separate PRs to help with reviewing.

Checklist

• Breaking changes are clearly marked as such in the PR description and changelog
• New behavior is reflected in tests
• If performance characteristic of an instruction change, update gas costs as well or make a follow-up PR for that
• The specification matches the implemented behavior (link update PR if changes are needed)

Before requesting review

• I have reviewed the code myself
• I have created follow-up issues caused by this PR and linked them here