Deprecate d3.geo.greatArc; add d3.geo.distance.

Now that D3 supports great-arc interpolation automatically for projections,
there is no need for a great arc feature generator, as a simple LineString
geometry object is equivalent. For the same reason, d3.geo.graticule does not
need to use supersampling on meridians, since meridians are great arcs.

This commit also adds d3.geo.distance(a, b) for computing the length of the
great arc between points a and b in radians, as a replacement for the
deprecated d3.geo.greatArc method, and a faster alternative to constructing a
d3.geo.interpolate solely to measure distance.

Fixes #1021 #1024.