Implement s2_rebuild_together()

[paleolimbot]

Right now s2_rebuild() treats all inputs as separate, which can result in problems when simplifying layers like polygon coverages and contour lines where creating new crossing edges would invalidate the layer topology. S2 provides a mechanism to rebuild multiple features together without creating new crossing edges which we should implement here: https://github.com/google/s2geometry/blob/master/src/s2/s2builder.h#L113-L117 .