Add angle-between-three-points function

[ianmackenzie]

Something like

Point2d.angleFrom :
    Point2d units coordinates
    -> Point2d units coordinates
    -> Point2d units coordinates
    -> Angle

and maybe also

Point3d.angleFrom :
    Point3d units coordinates
    -> Point3d units coordinates
    -> Point3d units coordinates
    -> Angle

where you give the start point, corner point and end point in order.