The title character of 'My Name Is Lucy Barton,' Rona Munro's crystalline stage adaptation of Elizabeth Strout's 2016 novel, is hardly a woman of mystery. On the contrary, as embodied with middle-American forthrightness by a perfectly cast Laura Linney, in the production that opened Wednesday at the Samuel J. Friedman Theater, Lucy may be the most translucent figure now on a New York stage.