Use Res instead of strings for associated items

The main idea is to, if given a `Self` type, skip over all the logic to find the root type and go straight to resolving the associated item. Everything else is just refactors to make implementing that possible.

- Switch to using `Res` instead of a string for `resolve_associated_item`
- Use a `Res` instead of a string for `variant_field`
- Remove `self_name`
- If given a `Self` type, skip over all the logic to find the root type
  and go straight to resolving the associated item
- Give a more helpful error if there's no `Self` type in scope
- Fix broken `all_fields()` logic - it should have been checking enum
variants instead

Author: jyn514