legion_prof_rs: Stack points to optimize selection of items for tiles

This MR changes the internal representation of Proc, Mem and Chan to stack points by level. This allows a more efficient implementation when we later consume the Proc, Mem or Chan points in a StateDataSource, making it more efficient to generate tiles.

Namely, the stacked representation maintains the invariant that on each level, items are non-overlapping and in sorted order, allowing us to use a binary search to find where a tile begins and ends.