In CLASS for OBJ find the index of the named SLOT.
The slot is a symbol which is installed in CLASS by the `defclass'
call. OBJ can be nil, but if it is an object, and the slot in question
is protected, access will be allowed if OBJ is a child of the currently
If SLOT is the value created with :initarg instead,
reverse-lookup that name, and recurse with the associated slot value.