"The power to tax the exercise of a privilege is the power to control and suppress its enjoyment. . . . A State may not impose a charge for the enjoyment of a Right granted by the federal constitution. . . . Thus it may not exact a license tax for the privilege of carrying on interstate commerce. . . . This tax is not a charge for the enjoyment of a privilege or benefit bestowed by the State. The privilege in question exists apart from State authority. It is guaranteed the People by the federal constitution."