“Leopoldstadt,” Tom Stoppard’s most personal play, is also true to the playwright’s form. It features a lot of talking, a surfeit of ideas, and is unrepentantly highbrow. It tells the story of a Jew-ish family at Vienna — there are baptisms and mixed marriages aplenty — between 1899 and 1955. Years of dashed dreams and cataclysm, on view at the Longacre Theatre.