1.0.7.26: asymptotically faster FIND-SLOT-DEFINITION

1.0.7.26: asymptotically faster FIND-SLOT-DEFINITION

 * Add a SLOT-VECTOR slot the SLOT-CLASS. The SLOT-VECTOR contains
   plists of slot definition objects hashed on the slot name, allowing
   O(1) lookups based on the slot name, instead of the old O(N)
   lookups -- where N is the number of slots in a class.

   Makes everything requiring slot definition lookups faster, simple
   tests showing SLOT-VALUE with variable name on a 5-slot class to be
   roughly 50% faster.