A year ago, David Schippers was preparing his final argument to a House Judiciary Committee considering articles of impeachment against President Clinton. His tone would be angry, even a little snide, he now admits, the work of a prosecutor sure of the president's guilt. Yesterday, in a speech and interview at Ashland University, Schippers looked back on his experiences as chief investigative counsel on the impeachment probe. His tone was wistful and even a bit peeved, the work of a prosecutor who still can't believe his target got away. "The system as developed by the framers of the Constitution worked ...