"There has been no clearer principle of English or American constitutional law than that, in criminal cases, it is not only the power and duty of juries to judge what are the facts, what is the law, and what is the moral intent of the accused; but that it is also their power, and their primary and paramount duty, to judge the justice of the law, and to hold all laws invalid, that are, in their opinion, unjust or oppressive, and find all persons guiltless in violating, or resisting the execution of, such laws."