Function for_each_interval 

fn for_each_interval<'a>(
    units: &'a [Unit],
    split_conflicts: &[Vec<usize>],
    max_input_files: usize,
    visit: impl FnMut(Range<usize>, &IntervalStats<'a>),
)

Visits every contiguous interval of units with at most max_input_files files: O(U · K) intervals for an input limit K. Each interval counts the conflicts of its split files from split_conflicts (see split_conflicts) with a binary search; an empty split_conflicts skips them.