Forty years ago, when U.S. cities began abandoning high-rise public housing, blasting crews would fill a tower with explosives and in a few monumental booms all would be reduced to rubble and rolling clouds of dust. It was as swift as it was symbolic. Now the demolitions are done by wrecking ball and crane, and the buildings are brought down bit by bit over months. This gradual dismantling seemed especially ill suited to the felling, in March 2011, of the last remaining tower at Cabrini-Green. Described almost unfailingly as “infamous” or “notorious,” this Chicago housing project had come to…