Am rejecting the patch because it violates the sort equivalence
guarantee (including sort's promise to maintain stability).

If you need the speed and don't care about sort stability, then just use
_heapq.nsmallest() or _heapq.nlargest() directly.

We could complexify the code a bit to achieve some automatic speed-up in
the case of key==None, but my timings don't show much of an improvement:

    if key is None:
        it = izip(iterable, count())                 # decorate
        result = _nsmallest(n, it)
        return map(itemgetter(0), result)            # undecorate
    else:
        in1, in2 = tee(iterable)
        it = izip(imap(key, in1), count(), in2)      # decorate
        result = _nsmallest(n, it)
        return map(itemgetter(2), result)            # undecorate