key provides C support for the decorate-sort-undecorate pattern.
  reverse provide a stable sort of the list with the comparisions reversed.

* Amended the docs to guarantee sort stability.

661092.

Armin Rigo's Draconian but effective fix for

SF bug 453523: list.sort crasher

slightly fiddled to catch more cases of list mutation.  The dreaded
internal "immutable list type" is gone!  OTOH, if you look at a list
*while* it's being sorted now, it will appear to be empty.  Better
than a core dump.

in the stability tests.

Bizarre:  this takes 11x longer to run if and only if test_longexp is
run before it, on my box.  The bigger REPS is in test_longexp, the
slower this gets.  What happens on your box?  It's not gc on my box
(which is good, because gc isn't a plausible candidate here).

The slowdown is massive in the parts of test_sort that implicitly
invoke a new-style class's __lt__ or __cmp__ methods.  If I boost
REPS large enough in test_longexp, even the test_sort tests on an array
of size 64 visibly c-r-a-w-l.  The relative slowdown is even worse in
a debug build.  And if I reduce REPS in test_longexp, the slowdown in
test_sort goes away.

test_longexp does do horrid things to Win98's management of user
address space, but I thought I had made that a whole lot better a month
or so ago (by overallocating aggressively in the parser).