Cleanup yielding a small speed boost: before rich comparisons were

Cleanup yielding a small speed boost:  before rich comparisons were
introduced, list.sort() was rewritten to use only the "< or not <?"
distinction.  After rich comparisons were introduced, docompare() was
fiddled to translate a Py_LT Boolean result into the old "-1 for <,
0 for ==, 1 for >" flavor of outcome, and the sorting code was left
alone.  This left things more obscure than they should be, and turns
out it also cost measurable cycles.

So:  The old CMPERROR novelty is gone.  docompare() is renamed to islt(),
and now has the same return conditinos as PyObject_RichCompareBool.  The
SETK macro is renamed to ISLT, and is even weirder than before (don't
complain unless you want to maintain the sort code <wink>).

Overall, this yields a 1-2% speedup in the usual (no explicit function
passed to list.sort()) case when sorting arrays of floats (as sortperf.py
does).  The boost is higher for arrays of ints.