Arena Stage’s The Fantasticks. The Fantasticks is a show already heavy on charm, but it never hurts to throw a little magic into the proceedings. That’s what Arena Stage has done with its current production of The Fantasticks, a 1960 show that was the world’s longest-running musical in its original off-Broadway run. You can feel the influence of The Fantasticks elsewhere in the musical theater canon, whether in Pippin’s title character’s search for his…