Neighborhoods in northeast Paris have struggled for years with the scourge of crack cocaine and its use in public. The Summer Olympics, kicking off a year from Wednesday, are offering an impetus to tackle the problem. Residents in the 18th and 19th arrondissements, or districts, of the French capital have long complained about the open-air crack use in their neighborhoods that stands in sharp contrast to the postcard-perfect tourist areas of Paris farther south.