Use ol.structs.RBush in ol.interaction.Modify

This is one more step to bring the master and vector-api
branches closer together. I am well aware that the current
Modify interaction will probably be rewritten as part of the
vector-api effort, but with changes like this we will eventually
be able to remove ol.structs.RTree from master.
Merge pull request #1322 from ahocevar/modify-rbush

Use ol.structs.RBush in ol.interaction.Modify