Many cities across southern China are known for the art of relaxing. Chengdu in Sichuan province is the tea house capital. Guangzhou is the birthplace of dim sum, a time to share steamed dumplings and chew the fat with friends. And in Haikou, the capital of Hainan province, people have been chewing the betel nut for centuries. You don’t have to walk far in Haikou to find a vendor. The small, hard, green fruits are sold in little piles alongside fresh coconuts and bottled water at pretty much any convenience store, for about five yuan (£0.52) a piece. Some vendors,…