WITH its long bulk of purple volcanic stone, the National Palace is not just any building. It looks out onto the Zócalo, the teeming heart of Mexico City, peopled by the dark-skinned Mexico that travels by bus or metro rather than in big SUVs with darkened windows. Tourists file in to the palace to see the murals by Diego Rivera. A less-visited corner houses the modest set of rooms where Benito Juárez, a 19th-century president, lived and died, having tackled the power of the Catholic church and defeated a French . . .