A stretch of Midtown Manhattan has become a “strip of despair,” where smacked-out addicts shoot up, light up and conk out at the feet of commuters and tourists, locals say. “I see a lot of things around here,” one shop owner told The Post of the so-called Eighth Avenue corridor near Penn Station, where there is a cluster of addiction clinics and homeless shelters. “Fights, drugs — oh my God — bad things. “I don’t know if they have knives or guns,” she said, explaining how people who appear to be both extremely high and severely disturbed regularly barge into...