Make a CubePoint instead of Point::s()

[leftiness]

CubePoint can have generic coordinate types so he can also replace FloatPoint::s(). Then I'll do something like this:

let CubePoint(q, r, s, t) = my_point.into();

Instead of this:

let FloatPoint(q, r, t) = my_float;
let s = my_float.s();