`World` is not cleared properly on demo restarts leading to super lag

[kungfooman]

You don't clean the world properly, that's why you collect subsystems and entries in the collision matrix etc.

However, you can easily add a method in World:

    clear() {
        const {bodies, constraints, solver, collisionMatrix, subsystems} = this;
        // Remove current bodies
        while (bodies.length) {
            this.removeBody(bodies.pop());
        }
        // Remove all constraints
        while (constraints.length) {
            this.removeConstraint(constraints[0]);
        }
        solver.removeAllEquations();
        collisionMatrix.matrix.length = 0;
        subsystems.length = 0;
    }

If this project weren't dead and written in JavaScript, I would have made a PR, but I just want to inform others about this leaky code.