Economist: ON A scrubby hillside in southern Uganda sit waist-high mounds of grass and twigs. They are the huts of Twa pygmies, the oldest residents of the Great Lakes region. Until recently they inhabited the nearby Bwindi Impenetrable Forest, but around 4,000 were expelled in 1991 after the forest was turned into a national park to protect its population of mountain gorillas. Now the pygmies languish outside it, unskilled at cultivating crops and often inebriated. For longer than history …