You might think a play that grapples with serious modern social issues — homophobia, teenage alienation, the limits of online privacy — would have no room for a warbling Abraham Lincoln doing an interpretive dance. But then you might not expect to encounter a piece of theater as ingenious and cannily plotted as Stephen Karam’s “Speech & Debate.” A suspenseful tale that fuses keen-eyed civic critique with riotous and even campy humor, Karam’s work is currently on view in a resonant and marvelously acted Rep Stage production, directed by Eve Muson.