Optimise d3.geo.area.

This incorporates two enhancements:

1. A simpler formulation for the area of a spherical triangle with one
   vertex at the south pole.
2. Instead of calling atan2 for every point, we use the identity:
     ∑ arg(z) = arg(∏ z),
   where z is a complex number, and only call atan2 once for each
   polygon, instead opting for a cheaper complex number multiplication
   for each point.

This is around 3x faster than the old implementation as measured by the
included benchmark.