Fix treatment of "self" when validating against trait method

If we're validating a class method against a trait method, we need
to treat "self" in the trait method as the class where the

Fix treatment of "self" when validating against trait method

If we're validating a class method against a trait method, we need
to treat "self" in the trait method as the class where the method
is used. To achieve this, we need to thread the proto scope through
all methods, so it can be provided separately from proto.common->scope.

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