Haymarket is Boston's great outdoor market, where you can buy everything from fruits and vegetable to sugar cane to fish just off the boat. It is everything your average supermarket isn't: cheap and loud and in your face. And don't even think of looking over any of those 12 limes you're about to spend a buck on. Squashed between a modern hotel and what is left of the Central Artery, Haymarket seems precariously perched on the edge of extinction. Even in a small city like Boston, Haymarket is tiny. And, indeed, some worry what will happen when the Big Dig - that giant project to put the Central Artery underground - finally takes away the land on which the two-day (Friday and Saturday) market now sits.