array.  Our samplesort special-cases the snot out of this, running about
12x faster than *sort.  The experimental mergesort runs it about 8x
faster than *sort without special-casing, but should really do better
than that (when merging runs of different lengths, right now it only
does something clever about finding where the second run begins in
the first and where the first run ends in the second, and that's more
of a temp-memory optimization).