Much of the cost of the `Relate` operation comes from finding all
intersections between all segments of the two geometries.

To speed this up, the edges of each geometry are put into an R-Tree.

We also have to "self node" each geometry, meaning we need to split any
segments at the point that they'd intersect with an edge from the other
geometry.

None of that work is new to this commit, but what is new is that we now
cache the self-noding work and the geometry's r-tree.

    relate prepared polygons
                            time:   [49.036 ms 49.199 ms 49.373 ms]
    relate unprepared polygons
                            time:   [842.91 ms 844.32 ms 845.82 ms]