On the surface,Chinatown is prosperous - a"model slum", some have called it - with the lowest crime rate,highest employment and least juvenile clelinquency of any city district.Walk through its crowded streets at any time of day,and every shop is doing a brisk and businesslike trade:restaurant after restaurant is booming;there are storefront displays of shiny squids,clawing crabs and clambering lobster;and street markets offer