"[A]ny inroad on presidential authority that would occur from prohibiting corrupt acts does not undermine the President's ability to fulfill his constitutional mission. The term "corruptly" sets a demanding standard. It requires a concrete showing that a person acted with an intent to obtain an improper advantage for himself or someone else, inconsistent with official duty and the rights of others. A preclusion of "corrupt" official action does not diminish the President's ability to exercise Article II powers."