Unity内部使用Quaternion来表示旋转，因为它比Matrix更精简，同时也没有Euler的gimbal lock问题。
Quaternion间的运算
transform.rotation *= target.rotation;
Quaternion与Vector3的运算
void Update()
{
Vector3 worldForward = transform.rotaion * Vector3.forward;
transform.position += worldForward * Time.deltaTime;
}
常用方法：
public static Quaternion LookRotation(Vector3 forward, Vector3 upwards = Vector3.up);
public static Quaternion FromToRotation(Vector3 fromDirection, Vector3 toDirection);
public static Quaternion Euler(float x, float y, float z);
public static Quaternion Slerp(Quaternion a, Quaternion b, float t);
public static float Angle(Quaternion a, Quaternion b);
