As Michelle Obama’s highly anticipated memoir “Becoming” arrives, it’s clear that the former first lady is occupying a space in the culture beyond politics. With an arena book tour featuring A-list special guests, she seems to exist in the middle ground between two icons she calls friends, Oprah Winfrey and Beyoncé Knowles-Carter. Her approach is short of Winfrey’s full-on confessional style but goes further than the guarded intimacy of Knowles-Carter’s art and performances.