More sort cleanup: Moved the special cases from samplesortslice into

listsort.  If the former calls itself recursively, they're a waste of
time, since it's called on a random permutation of a random subset of
elements.  OTOH, for exactly the same reason, they're an immeasurably
small waste of time (the odds of finding exploitable order in a random
permutation are ~= 0, so the special-case loops looking for order give
up quickly).  The point is more for conceptual clarity.
Also changed some "assert comments" into real asserts; when this code
was first written, Python.h didn't supply assert.h.