use vec3 for cuboid shape

spatial/wit/wired-physics/world.wit

@@ -15,16 +15,10 @@ interface types {
   }
 
   variant shape {
-    cuboid(cuboid),
+    cuboid(vec3),
     sphere(sphere),
   }
 
-  record cuboid {
-    x-len: f32,
-    y-len: f32,
-    z-len: f32,
-  }
-
   record sphere {
     radius: f32
   }