quick_sort returns incorrect results

[brson]

std::sort::quick_sort sorts [2, 1, 3] as [1, 3, 2], for both the vec and ivec versions.

fn test_qsort() {
    auto names = ~[mutable 2, 1, 3];

    auto expected = ~[1, 2, 3];

    fn lteq(&int a, &int b) -> bool { int::le(a, b) }
    iqsort(lteq, names);

    auto pairs = ivec::zip(expected, ivec::from_mut(names));
    for (tup(int, int) p in pairs) {
        log_err #fmt("%d %d", p._0, p._1);
        assert p._0 == p._1;
    }
}

[msullivan]

The problem is that if the pivot element winds up at spot zero, we return early, without sorting the right side. Fix incoming.