Handle (aref v (+ i k)), with i negative

Handle (aref v (+ i k)), with i negative

 * Update the fndb to allow negative index values for foo-ref-with-offset
   and foo-set-with-offset.

 * Adjust VOPs accordingly.

 * Fix fold-index-addressing: only fold constant offsets if the resulting
   index argument would be a fixnum, and compute the new offset correctly
   for subtractions.

 * Unmark the corresponding test as an expected feailure, and add a test
   to make sure VOPs for data-vector-{ref,set}-with-offset  accept negative
   index values (unless the element size is too small
   to fold offsets in an EA).

 * Un-package-qualify a few spurious test-util:with-test.