rustdoc: Add support for associated items even outside the impl itself

Previously, associated items would only be available for linking
directly on the `impl Trait for Type`. Now they can be used anywhere.

- Make `item` for resolve mandatory
- Refactor resolving associated items into a separate function
- Remove broken trait_item logic
- Don't skip the type namespace for associated items
- Only call `variant_field` for `TypeNS`
- Add test for associated items
- Use exhaustive matches instead of wildcards

Wildcards caused several bugs while implementing this.

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