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 (b114b4d) with master (1bdd2ce)}

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.