Function split_for_merge 

fn split_for_merge<T, F>(left: &[T], right: &[T], is_less: &F) -> (usize, usize)
where
    F: Fn(&T, &T) -> bool,

Splits two sorted slices so that they can be merged in parallel.

Returns two indices (a, b) so that slices left[..a] and right[..b] come before left[a..] and right[b..].