EXCLUSIVE: Elle Fanning and Diego Luna have joined the cast of Clay Jeter’s sci-fi pic IO, a project that was developed at the Sundance Institute’s Writers Lab and Sundance Catalyst Forum. Jeter, who made the 2011 Sundance Film Festival pic Jess + Moss, wrote the script with Will Basanta and Charles Spano. The character-driven IO, slated to begin production sometime this year, tells the story of a girl's coming of age while examining the dangers of humanity's current…