Rtree state methods

I was planning to add a #state method returning an object corresponding to the state_t struct, but that is allocated and deallocated from rtree_new, rtree_destroy so one would need to jump through some hoops to stop the Ruby GC from deleting the C rtree when the state is still in use. In the end easier to just have these methods against the RTree instance instead (and the fact that we store dimension etc in a state_t is an implementation detail, not of interest to the user).