PANAMA captures the sweep of this country's tumultuous history.  Moving between past and present -- from the pivotal events in Panama's past to the American withdrawal in 1999 -- this is the fast-paced saga of a man and a nation searching for their identities.     Hank Duque is a third-generation Canal Zonian, a hybrid of American and Panamanian ancestry, who returns to Panama in 1989, after a twenty-five-year exile.  Immediately upon his arrival, a La Guardia intelligence officer, Major Carlos Mejas, recognizes Hank and knows of his role in the bloody 1964 student riot.  Hank wonders how the major was able to identify him so quickly.     Hank has an assignment to write the Panama section of a Central American travel guide.  He plans to be in the country no more than a week.  However, he meets Andrea Arias, a self-reliant Panamea, and becomes caught up in the prevailing political upheaval as Manuel Noriega, a former CIA operative and now military dictator, defies both the United States government and the Panamanian people.       Hank's odyssey takes him throughout Panama, from the teeming streets of Panama City to the impenetrable jungle of the interior, from the landscaped boulevards of the Canal Zone to a squalid cell known as La Preventiva.       "Sal si puedes," Hank's Panamanian grandfather had implored.  "Get out if you can."       Now, twenty-five years later, Hank has returned.