Fixes missing error on type unification.

[esdrubal]

Description

While using generic blanket implementation no error was thrown when another type other than the generic blanket expected type was used.

This was caused by the typechecker being too permissive in UnknownGeneric unification. The fix changes the unifier to handle unification with the expected self and generics as special cases.

Closes #5212
Closes #5162

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).
• I have added tests that prove my fix is effective or that my feature works.
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• I have done my best to ensure that my PR adheres to the Fuel Labs Code Review Standards.
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[IGI-111]

If we're going to have multiple modes like this, it might be a good idea to refactor the unifier to work like UnifyCheck.

[esdrubal]

If we're going to have multiple modes like this, it might be a good idea to refactor the unifier to work like UnifyCheck.

The Unifier so far only has to handle the self and generic special cases which can be contained in the same match expression with a simple conditional check in the respective match arm.

[esdrubal]

Added test showing that #5162 is resolved with the changes proposed in this PR.