Robyn Eckhardt writes:  In today's Wall Street Journal Asia 'Weekend Journal' we take you for a stroll along the soi (alleys) of Chiang Mai's moat-encircled Old Town and to Penang, in search of the story behind one of the Malaysian state's iconic dishes: assam laksa. For us, the highlight of the former is lunch at Huen Phen, a restaurant specializing in northern Thai dishes. (Go during the day when lunch is served in the casual front of the restaurant, flavors are authentic, and the place is packed with locals. Come evening meals are served in the restaurant's rear; though the decor is lovely, dishes are decidedly toned down to cater to farang palates.)...