update ecs package

[yohamta]

Simplify the API.

Example:

func NewGame() *Game {
	// ...
	g.ecs.
		// Systems
		AddSystem(system.NewSpawn().Update).
		AddSystem(metrics.Update).
		AddSystem(system.NewBounce(&g.bounds).Update).
		AddSystem(system.Velocity.Update).
		AddSystem(system.Gravity.Update).
		// Renderers
		AddRenderer(layers.LayerBackground, system.DrawBackground).
		AddRenderer(layers.LayerMetrics, metrics.Draw).
		AddRenderer(layers.LayerBunnies, system.Render.Draw)
}

func (g *Game) Update() error {
	g.ecs.Update()
	return nil
}

func (g *Game) Draw(screen *ebiten.Image) {
	screen.Clear()
	g.ecs.DrawLayer(layers.LayerBackground, screen)
	g.ecs.DrawLayer(layers.LayerBunnies, screen)
	g.ecs.DrawLayer(layers.LayerMetrics, screen)
}