1.0.4.11: trivial hash-table cleanup / optimization

1.0.4.11: trivial hash-table cleanup / optimization

 * Hash table code does lots of AREFs using indexes (* 2 i) and (1+ (* 2 i)),
   where I obviously has to be less then (floor array-dimension-limit 2),
   for which both calculations are known to fit into a fixnum.

 * Define such a type (aka INDEX/2) and use it where appropriate. We
   were already doing this in WITH-HASH-TABLE-ITERATOR, but not
   elsewhere.