[WIP] Go to new sparse packs

[Yurlungur]

PR Summary

Refactor to new type-based indexes, and also coalesce kernels

PR Checklist

• Adds a test for any bugs fixed. Adds tests for new features.
• Format your changes by calling scripts/bash/format.sh.
• Explain what you did.

[Yurlungur]

This is too much stuff at once. I'm going to break this out into several PRs.