Improve QuadPath tree storage structure

Currently, when elements are split, the subelements are stored in a newly allocated child QuadPath object. This is needs to be improved because currently

1. It causes a lot of memory allocations and pointer chasing
2. There is no memory management, so every split path is leaked
3. It's wasteful and confusing since a QuadPath object contains meta information that is not relevant for a single curve's subelements