Function partition 

fn partition<T, F>(v: &mut [T], pivot: usize, is_less: &F) -> (usize, bool)
where
    F: Fn(&T, &T) -> bool,

Partitions v into elements smaller than v[pivot], followed by elements greater than or equal to v[pivot].

Returns a tuple of:

1. Number of elements smaller than v[pivot].
2. True if v was already partitioned.