Time tugs strangely on the sleeve of “Sweet Thing,” a heartfelt, hopeful yet slightly hollow black-and-white coming-of-ager from American indie stalwart Alexandre Rockwell (“In the Soup,” “Pete Smalls Is Dead”). A lively, bittersweet meditation on an impoverished childhood that is still rich in innocence and imagination, it feels old-fashioned in a way that does not quite gel with its bid for contemporary grit.