Renaissance Technologies, the giant hedge fund founded by billionaire Jim Simons, looks like it has created the premier, tax-free retirement account for its employees. A deft maneuver back in 2012 enabled employees to invest in RenTech’s storied Medallion Fund via individual retirement accounts, or IRAs, which provide tax benefits for millions of ordinary Americans savers who are putting money aside for their golden years. The difference: ordinary people don’t have access to Medallion, one of the most successful hedge funds of all time.