In “And So We Walked,” playwright and performer DeLanna Studi recounts an early memory. The child of a white mother and Cherokee father, her father once told her, “You will be a bridge between your mother’s people and mine.” Instead, she tells us, she’s more of an island between two shores. “And So We Walked” is Studi’s first play. It’s getting its premiere at Portland Center Stage, where she recently starred in Astoria. Throughout the autobiographical performance, Studi explores her relationship with her father and her Cherokee heritage. This culminates in a six-week, 900-mile trip along the Trail of Tears that she persuades her father to take with her.