fix: Make trait functions generic over Self

[jfecher]

Description

Problem & Summary

Checking a trait function's type previously was very ad-hoc. When a TraitMethodReference was found, it'd manually create a function type from the trait function's definition and instantiate it - even though instantiating a Type::Function does nothing.

I've changed the TraitFunction struct to add a Type field for the whole function type, which includes generics. The type now should usually be a Type::Forall. In addition, I've added the trait's Self type to the list of generics in the Type::Forall.

This fixes the issue where the Self type would be bound after certain function calls which would affect callsites but not a function body leading to a type mismatch between function return type and a call's return type. This is the issue we were encountering after adding traits to noir-protocol-circuits.

Additional Information

No tests are provided since I was unable to make a minimal repro from the thousands of lines of noir-protocol-circuits unfortunately. I'll keep experimenting while this PR is up.

I'm also merging into jf/fix-3089 since this was somewhat accidentally built on top of it. The two changes are otherwise unrelated.

Documentation*

Check one:

• No documentation needed.
• Documentation included in this PR.
• [Exceptional Case] 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.