Comic book mavens rejoice, for Marvel Studios has concocted a heady and wonderful sensory brew in “The Avengers.” Weaving together the origin stories of Iron Man, Thor, Hulk, and Captain America, “The Avengers” packages a star-studded cast, one that could easily have imploded under its own weight, into a fleet-footed, yet cohesive, plotline. Nefarious forces from other worlds threaten Earth, led by the sneering and magnetic trickster god