Halfway through David Means’s brilliant new novel, HYSTOPIA (Farrar, Straus and Giroux, $26) — a careening metafiction that hallucinates a post-Vietnam America governed by a third-term JFK in which gangs of Black Flag bikers rove the state of Michigan — a young woman named Meg, who has been sprung from psychiatric care by a deranged killer named Rake, submerges herself in cold water. She’s trying to reverse the effects of “enfolding,” an extreme form of therapy popular with returning soldiers that annihilates traumatic memories. (Other than this secular baptism, the only means of “unfolding” is…