Optimise area calculation for CCW detection.

Note that D3 currently uses NASA’s approximation algorithm to detect CCW polygons (combined with a latitudinal rotation for other reasons), so TopoJSON is already using a different (more precise but slower) algorithm for CCW detection.  However, I plan on replacing D3’s CCW detection with this faster algorithm very soon (as well as dropping the latitudinal rotation), so both libraries will be completely in sync.

As well as being faster, I believe this new algorithm is also more accurate.