Change implementation of MERGE-SORT (and therefore SORT) so that it

Change implementation of MERGE-SORT (and therefore SORT) so that it
uses the in-place vector-sorting algorithm for lists.  The previous
algorithm created a stack of depth (/ (LENGTH L) 2), which made it
impossible to use for large lists.  This algorithm creates a stack of
depth (/ (LOG (LENGTH L)) (LOG 2)).

Additionally, tweaked the vector-sorting algorithm to use indexes in a
slightly more efficient (and clearer) way.