Previously, TcCoercion were only used to represent boxed Nominal coercions. In order to also talk about boxed Representational coercions in the type checker, we add Roles to TcCoercion. Again, we closely mirror Coercion. The roles are verified by a few assertions, and at the latest after conversion to Coercion. I have put my trust in the comprehensiveness of the testsuite here, but any role error after desugaring popping up now might be caused by this refactoring.
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