In this age of calculatedly incalculable careerism, Emma Stone still doesn’t have a five-year plan. But at 23, she’s already poised to become one of her generation’s most romantically comedic—and remarkably versatile—box-office powerhouses. Talking Spidey-induced mayhem, hysterical laughter, tears of joy, and the sublime Zen of Bill Murray with the actress who is improvising her way to the movie-star bank