Returns a list of 'standard-direct-slot-definition'
objects for a class and its subclasses. Slots objects for slots that
do not have reader accessors are filtered out and not returned.

If 'readablep' is true, filters out the slots that don't have a
reader (or accessor) defined.
If 'writablep' is true, filters out the slots that don't have a
writer (or accessor) defined.