Check for type aliases when implementing traits

[jjcnn]

Description

Fixes #6329 .

#6867 makes a lot of changes to TraitMap::insert and the unification that happens inside the trait map, so I'm postponing this issue until that PR is in a stable state.

Status:

• Testcase added. The compiler correctly rejects the program, but the error message is unhelpful. The error message suggests that the alias still is treated as different type from the aliased type, which ought to be fixed.

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).
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• I have added tests that prove my fix is effective or that my feature works.
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