feat: error if generic type parameter in impl is not mentioned in self type

[asterite]

Description

Problem

Resolves #5855

Summary

I decided to do this check separately... that is, another way could have been to keep track of which generics we saw while we solve self_type, but I think that would lead to code that's harder to understand. I was also able to use the visitor pattern to quickly find the mentioned generic types (I had to update Visitor so that it correctly traverses unresolved type expressions).

Additional Context

Some questions:

1. We only produce this error for impl... do we also want to do it for trait impls? If yes, in this PR or a separate one?
2. Should we use the same error message as Rust? I find it a bit hard to understand, although it kind of makes sense. Let me know!

Documentation

Check one:

• No documentation needed.
• Documentation included in this PR.
• [For Experimental Features] Documentation to be submitted in a separate PR.

PR Checklist

• I have tested the changes locally.
• I have formatted the changes with Prettier and/or cargo fmt on default settings.

[asterite]

Marked as draft because of #5855 (comment)