Provide an interface for `RuleSet|CSSList|Import|Charset`

[oliverklee]

This will help us reduce some overlong type annotations in CSSList.

[JakeQZ]

Could we merge all these into one? Having a half a dozen separate issues clogs up the log.

EDIT: Please ignore that comment. I meant "search results" not "log". The issue I was having was searching issues, and finding all six of these similar issues on the first page of results. They do all need to be done on a case-by-case basis.