quadtree: implement support for C++20 std::function predicate
quadtree: implement support for C++20 std::function predicate

Useful if you have limited search criteria for the Quadtree and want to
stop iteration early.

Signed-off-by: Steven Noonan <4068f0880b399410602d694b3cc711c8a8f4727e@uplinklabs.net>

tycho/crisscross,tycho/crisscross,tycho/crisscross