CUPERTINO, Calif. — Apple Inc. unveiled a new iPhone on Tuesday that is faster and more powerful but stops short of a more radical upgrade. It said Sprint customers will now be able to use one. The new iPhone 4S has an improved camera with a higher-resolution sensor. The processor is faster, which helps run smoother, more realistic action games. It’s also a “world phone,” which means that Verizon iPhones will be able to useable overseas, just as AT&T iPhones already are.