Today’s Opinionist comes to DCist from local art blogger Kriston Capps. For all this time, D.C. Mayor Williams has billed himself as a supporter of big boxes in the District. During yesterday’s town hall meeting to discuss the fate of the city’s public library system, the Mayor revealed himself to be no friend to our most notorious big box—the Mies Van der Rohe-designed Martin Luther King Jr. Memorial Library. And he made no new friends…