move: implement circle movement

vec.ts

@@ -13,6 +13,10 @@ export default class Vec2d implements Point {
     this.y = y;
   }
 
+  static fromPolar(r: number, theta: number) {
+    return new Vec2d(r * Math.cos(theta), r * Math.sin(theta));
+  }
+
   static random(maxX: number, maxY: number) {
     return new Vec2d(Math.random() * maxX, Math.random() * maxY);
   }