Merge #693

Merge #693

693: Name resolution refactoring r=matklad a=flodiebold

This is still very WIP, but it's becoming quite big and I want to make sure this isn't going in a completely bad direction :sweat_smile:. I'm not really happy with how the path resolution looks, and I'm not sure `PerNs<Resolution>` is the best return type -- there are 'this cannot happen in the (types/values) namespace' cases everywhere. I also want to unify the `resolver` and `nameres` namespaces once I'm done switching everything to `Resolver`. Also, `Resolver` only has a lifetime because it needs to have a reference to the `ItemMap` during import resolution :confused:

The differences in the completion snapshots are almost completely just ordering (except it completes `Self` as well now), so I changed it to sort the completions before snapshotting.

Co-authored-by: Florian Diebold <flodiebold@gmail.com>