In 1987, David Lang was a 30-year-old composer and doctoral student who, with his Yale buddies Michael Gordon and Julia Wolfe, founded Bang on a Can, a scruffy organization dedicated to the proposition that all musics are created equal. These days, Lang is an eminence: Pulitzer Prize winner, member of the Yale faculty, and composer in residence at Carnegie Hall for 2013-14. Justin Davidson talked with him midway through “collected stories,” a six-concert festival he curated at Carnegie’s Zankel Hall, and days before the release of his recordinglove/fail…