It's a long, long way to Britain from Key West, Florida - via Mexico and Alberta - on a bike. But Bobby MacArthur, who has done just that, looked up from feet-somethin " Bobby was on the last gasp of the second year of a three-year spin on the fabulous contraption he calls a bicycle. "I'm doing it for a little wager I had with two guys back home," he said. The "little wager,"he added, was a "mere" 10,000 dollars with a few side bets amounting to another 3,000. And the bike! Well, there : we were