Harare, Zimbabwe — Zimbabwe’s recently introduced gold-backed currency is sliding on the local black market but officials insist the currency is getting stronger and has a bright future. Columbus Mavhunga reports from Harare. Even songs are played on the radio encouraging citizens to embrace the currency, called Zimbabwe Gold — or ZiG — introduced on April 5 trading at 13.56 to the U.S. dollar. Official statistics say ZiG is now trading at 13.41. But on the black market it is around 20. Chamunorwa Musengi, a street vendor in Harare, is not …