Consider all primary methods when calculating effective dispatch value

[camsaul]

Consider this case:

• A :toucan isa :can
• A :toucan isa :bird
• We have a preference for :bird over :can
• We have a primary method for :can
• We have a primary method for :bird

If you invoke the multimethod with dispatch value :bird, it will use the :bird primary method with no next-method.

If you invoke the multimethod with dispatch value :toucan, it will use the :bird primary method with the :can primary method as the next-method.

Thus even though dispatch values :bird and :toucan both use the :bird primary method, their effective dispatch values are not the same; because invoking the method with dispatch value :bird and with dispatch value :toucan is not the same thing.

We need to consider all matching dispatch values when calculating effective dispatch values.

This PR fixes the bug above