WASHINGTON — Justice Stephen Breyer on Tuesday said liberal advocates of big changes at the Supreme Court, including expanding the number of justices, should think “long and hard” about what they’re proposing.Politically driven change could diminish the trust Americans place in the court, Breyer said in the prepared text of a long speech he gave remotely Tuesday to Harvard Law School students, faculty and alumni.His talk, Breyer said, “seeks to make those whose initial instincts may favor important structural (or other …