Add circle_fixture for use B2DCircleShape

Add circle_fixture to using circle body.

    ball_body = @world.new_body(
      position: [Screen.half_width, Screen.half_height],
      type: KDynamicBodyType
    ) do
      circle_fixure(
        radius: 100,
        friction: 0.6,
        restitution: 0.6,
        density: 2.0
      )
    end
This code requires this pull-request.
https://github.com/CurveBeryl/Joybox-Box2D/pull/2
You need to merge it and build Box2D.framework before merge this request.