Finish the struts

Maybe you already question yourself if there is something similar to std::geo2d::Frame() for our circles?

And yes there is, and it's called std::geo2d::Ring. So let's shorten our strut code a last time:

use std::geo2d::*;
use std::ops::*;

Ring(outer_d = 6.51mm, inner_d = 4.8mm).translate(x = [-1..1] * 8mm);

At this point, we are almost finished with the base. We just have to find a way to combine frame and structs.