Clean up the quicksort API

[SimonSapin]

• Change quicksort_by to take a closure (this should work now that they’re unboxed) instead of an fn
• Add a quicksort function:

#[inline]
fn quicksort<T>(arr: &mut [T]) where T: Ord {
    quicksort_by(arr, |a, b| a.cmp(b))
}