Add hal::soft::sync_servo

[kammce]

class sync_servo_adapter {
  sync_servo_adapter(hal::servo& p_servo, hal::rotation_sensor& p_sensor);
};

When the position() API is called, it blocks until the rotation sensor reaches the desired position. This is a utility adaptor for when an application wants to wait until the servo has reached its destination before proceeding to the next step of the movement.