Can't provide an implementation for an :abstract output as a mixin (DEFEDIT-1483)

Say we have a defnode A with an abstract output O, and a defnode C with a concrete implementation for O. A defnode D inheriting from A and O will complain that O is still abstract. This is sorta kinda how most OO languages would work, but our "inherits" semantics is more of "inject the definition of X here" so I would expect D to have C's implementation of O in place.