Fixes insert_trait_implementaion for nested generics.

[esdrubal]

Description

The unify_checker was returning false when comparing MyOption<T> with T. And filter_by_type_inner type substitution when adding methods from MyOption<T> to MyOption<MyOption<T>> was replacing MyOption in its own type, ending up by inserting into type MyOption<MyOption<MyOption<T>>> instead of MyOption<MyOption<T>>.

Fixes #6825

Checklist

• I have linked to any relevant issues.
• I have commented my code, particularly in hard-to-understand areas.
• I have updated the documentation where relevant (API docs, the reference, and the Sway book).
  • If my change requires substantial documentation changes, I have requested support from the DevRel team
• I have added tests that prove my fix is effective or that my feature works.
• I have added (or requested a maintainer to add) the necessary Breaking* or New Feature labels where relevant.
• I have done my best to ensure that my PR adheres to the Fuel Labs Code Review Standards.
• I have requested a review from the relevant team or maintainers.

[codspeed-hq]

CodSpeed Performance Report

Merging #6827 will not alter performance

_{Comparing esdrubal/6825_nested_generics2 (70087aa) with master (1192b3f)}

Summary

✅ 22 untouched benchmarks

[jjcnn]

The changes are fine. I just don't understand why the code that's being removed was introduced in the first place.

I've added a nit about the length of one of the methods. Another nit is this:

The unify_checker was returning false when comparing `MyOption<T>` with `T`. 

Surely those two types don't unify (unless the two Ts are independent type variables)? AFAICT the test you've introduced also doesn't test that unification.

[jjcnn]

It seems reasonable to remove this bit of code, but why was it there in the first place? It seems like an odd thing to write by accident, so surely there was a point to it?

[jjcnn]

Nit: We really should refactor this method. It's extremely long, and it's hard to follow what's going on when the entire method doesn't fit on the screen.