“Rafiki,” which comes out on Friday, was directed by Wanuri Kahiu. The Nairobi-born director (b. 1980) has just one other feature-length film under her belt, “From a whisper.” She’s new on the scene, young(er) and taking risks.In “Rafiki” Kena (Samantha Mugatsia) and Ziki (Sheila Munyiva) eye each other from across the street, with a mixture of suspicion and attraction. The former is thin like a reed and reserved and surveys