"Statutory defenses. ...No principle of statutory construction justifies narrowing... to cover only conduct that impairs the integrity or availability of evidence. Sections 1503 and 1505... offer broad protection against obstructive acts directed at pending grand jury, judicial, administrative, and congressional proceedings, and they are supplemented by a provision in Section 1512(b) aimed specifically at conduct intended to prevent or hinder the communication to law enforcement of information related to a federal crime."