"Whether a law be void for its repugnancy to the Constitution, is, at all times, a question of much delicacy, which out seldom, if ever, to be decided in the affirmative, in doubtful case. ... But it is not on slight implication and vague conjecture that the legislature is to be pronounced to have transcended its powers, and its acts to be considered as void. The opposition between the Constitution and the law should be such that the judge feels a clear and strong conviction of their incompatibility with each other."