加一个函数，使结果可视化

加一个函数，打印出来更好看

func (car *Car) String() string {
    return fmt.Sprintf("{%s,%s,%d}", car.Model, car.Manufacturer, car.BuildYear)
}

打印结果：

AllCars: [{Fiesta,Ford,2008} {XL 450,BMW,2011} {D600,Mercedes,2009} {X 800,BMW,2008}] New BMWs: [{XL 450,BMW,2011}]

Map sortedCars: map[Aston Martin:[] BMW:[{XL 450,BMW,2011} {X 800,BMW,2008}] Default:[{D600,Mercedes,2009}] Ford:[{Fiesta,Ford,2008}] Jaguar:[] Land Rover:[]] We have 2 BMWs