Somewhat-functional merge implementation.

Because of the necessary point-in-polygon check to reconstruct polygon systems
after merging, this implementation currently assumes Cartesian coordinates.
However, in many cases the coordinate system should not have any effect on
merging, so I’m hoping this isn’t too severe of an initial limitation.

Also, fix two bugs in topojson.mesh: the `start` and `end` temporaries on each
arc weren’t being cleaned up after the mesh was constructed, and we were
assuming delta encoding rather than checking for a transform.