s2: Add ChildBegin/ChildEnd family of methods.

Signed-off-by: David Symonds <dsymonds@golang.org>
s2: Add Cell type.

Add a couple of methods to s2.CellID that are needed for that.

Signed-off-by: David Symonds <dsymonds@golang.org>
s2: Add PointArea function for computing the area of a triangle.

	BenchmarkPointArea              10000000        240 ns/op
	BenchmarkPointAreaGirardCase    10000000        275 ns/op

Signed-off-by: David Symonds <dsymonds@golang.org>