Allocate additional space for class unique number and class slots.

Since the unique number of the class and the class slots of the class
are no longer treated as slots, we need to handle them specially by
allocating additional space for them in the contents vector.
Removed slot INSTANCE-SLOTS from STANDARD-OBJECT.

We store a copy of the effective slots of the class in the instance of
STANDARD-OBJECT as before, but we no longer treat the place where this
information is stored as an ordinary slot.  Instead, in a standard
object, it will be stored at offset 1, right after the unique number
of the class.