Improvements to the :ARRAY type.

- Got rid of the :AUTO-ARRAY type. Add translators to :ARRAY
  directly instead.
- Make the :ARRAY type aggregate. This way it can be used
  inside structures as an alternative to the :COUNT slot option.
- Add TRANSLATE-AGGREGATE-TO-FOREIGN method that lets us do
  (setf (foreign-slot-value ptr 'struct 'slot-name) #(1 2 3))