AMONG THE MANY measures and half-measures that are being proposed to solve the crisis of illegal immigration, there have been some real doozies: a 700-mile wall to keep people out (or in?); a temporary guest-worker program that may end up harming both American and Mexican employees; even a scheme for the largest mass deportation in U.S. history. But here's one good idea you won't hear about. Let's allow the North American Free Trade Agreement to live up to its promise and permit citizens of Canada, the United States and Mexico to move and work freely among the three countries. ...There...