Fix winding order detection for large polygons.

The trick to avoid atan2 calls fails for winding order detection in the
case where a counter-clockwise area strays into a positive quadrant
(similarly for a clockwise area straying into a negative quadrant).
Optimize topojson.neighbors.

We now build up the map from arc to object first in its entirety, rather than
incrementally building the map from object to object. Since most arcs are only
used by one or two objects, this is much faster. Computing the neighbors from a
collection of 3,641 U.S. counties previously took about 1.3 seconds, and now
takes about 15 milliseconds, and improvement of almost two orders of magnitude.
Bump version number.