refactor: separate ChainStore and ChainIndex

[lemmih]

Summary of changes

Changes introduced in this pull request:

• Move tipset_from_keys from ChainStore into ChainIndex. This is where it belongs, conceptually.
• I'll rewrite all of ChainIndex in a follow-up PR.

Reference issue to close (if applicable)

Closes

Other information and links

Change checklist

• I have performed a self-review of my own code,
• I have made corresponding changes to the documentation,
• I have added tests that prove my fix is effective or that my feature works (if possible),
• I have made sure the CHANGELOG is up-to-date. All user-facing changes should be reflected in this document.

[LesnyRumcajs]

It'd be fantastic to take this opportunity to write some high-level doc in those modules on what they do, conceptually.

[lemmih]

It'd be fantastic to take this opportunity to write some high-level doc in those modules on what they do, conceptually.

It's coming in a follow-up PR.