It’s been called a “game changer,” a “world changer” and a “life changer.” It’s predicted to disrupt everything from medicine to manufacturing to military strategy. It’s being used to create guns and gold jewelry, houses and human hearts. Three-D printing has evolved in the past few years from an obscure industrial process to an exemplar of digital-age innovation — with all the thrilling potential, lurid hype and amorphous menace that goes with it.