Boston's august museum has an enviable Americas collection: everything from Maya ceramics to Paul Revere silver to Warhol silk screens. Now it finally has enough room to show it off properly. Foster + Partners has provided a lucid, spacious new addition, with 53 galleries and a glass-enclosed courtyard. And the art of Central and South America is now installed in close proximity to art of the U.S., weaving the cultural history of two continents into a single story. 11/20