If no After function is provided for a SwitchingCard, it uses ShuffleIntoBottomHalf.
This also modifies Then to be callable with a nil `o`, which it recognizes as "nothing happened but we need cleanup anyway", in preparation for the incoming "make cards sometimes not drawable" feature.
BasicPolicy doesn't know how to do fallback, that's driven by the VerbosePolicy. But the VerbosePolicy isn't there if BasicPolicy is trying to use CanDo itself. Just remove the check.
This allows it to switch between other complete policies, with fallbacks to a default if parts aren't impelemented. Complementing it is the change to BasicPolicy, which throws ErrUnimplemented if fields are missing, which VerbosePolicy uses as a "go ask the default" sign.
