Remove successive coincident arcs from rings.

Quantisation can introduce coincident arcs.  If this occurs during
pre-quantisation, it can result in awkward topologies, violating
assumptions made by topojson.mergeArcs.

Ideally we’d like to remove all coincident arcs from polygons, including
those generating when quantising an isthmus (which splits a ring into
two).  Removing successive coincident arcs should fix the most common
cases for now, and doesn’t require splitting rings.

Related discussion in #164.