PARK CITY, UTAH – The last documentary film that used dry charts and statistics to make an abstract argument about a global issue and nonetheless became a pop-culture hit was Al Gore’s An Inconvenient Truth. But the hit of this year’s Sundance Film Festival was a low-key affair called Inequality for All, in which Robert Reich, a labor secretary in the Clinton administration, explains how rising income inequality and the demise of the middle class is causing so many Americans to suffer.