feat: Improve performance of stack function (#1281)

* Initial attempt at a fast stacking algorithm

This one only works if the items are sorted by their start time. So unfortunately it doesn't play nice with custom ordering.

* Working fast stacking algorithm

Now the stacking algorithm respects order correctly, while still making a significant improvement to performance.

* Further performance improvements to stacking algorithm

Co-authored-by: Joshua Walsh <josh.walsh@carevision.com.au>