LONDON — The ducks, geese and hardy cold-water swimmers in London’s Hyde Park have a new neighbor: a monumental floating structure made from 7,506 stacked barrels colored bright red, mauve and blue. “The London Mastaba,” unveiled Monday, is the latest installation by Christo, a master of supersized artworks who has previously wrapped Berlin’s Reichstag in silver fabric and festooned New York’s Central Park with thousands of saffron-colored cloth gates. The 83-year-old artist’s first major creation in London rises 65 feet above the surface of the park’s Serpentine Lake.