Rewrite the current range-tree API used by dyn-topo undo
to avoid inefficiencies from stdc++'s set use.
- every call to `take_any` (called for all verts & faces)
removed and added to the set.
- further range adjustment also took 2x btree edits.
This patch inlines a btree which is modified in-place,
so common resizing operations don't need to perform a remove & insert.
Ranges are stored in a list so `take_any` can access the first item
without a btree lookup.
Since range-tree isn't a bottleneck in sculpting, this only gives minor speedups.
Measured approx ~15% overall faster calculation for sculpting,
although this number time doesn't include GPU updates and depends on how
much edits fragment the range-tree.
