Port ol.source.Vector to ol.structs.RBush
Port rtree example to ol.structs.RBush
Move ol.structs.RTree out of the way
Add ol.structs.RBush#assertValid
Refactor ol.structs.RBush tests
Don't check for under-full nodes when validating ol.structs.RBush

Removal can lead to under-full nodes.
Implement ol.structs.RBush#remove_
Add FIXME
Add ol.structs.RBush#update
Keep R-Tree up-to-date in ol.source.Vector