Officially, Nigeria’s naira is doing pretty well this year, down just 4% against a strong US dollar, ahead of the Canadian dollar and the Swiss franc. But that exchange rate is used only by the government -- ordinary Nigerians are grappling with a 37% drop on the widely-used black market. That makes it one of the world’s worst-performing currencies, after Ghana’s cedi, which is down nearly 55% this year, and the Sri Lankan Rupee. Its peers include Sierra Leone’s leone, which is down 36%, and the Egyptian pound, which has lost 35%. Africa’s largest economy operates a tightly controlled official