In 1908, Americans mailed more than 677 million postcards — that’s a lot of wish-you-were-heres. The Boston Public Library’s “Greetings From Boston’’ is a colorful exhibit of vintage postcards that show off the architecture and cultural history of early-20th-century Boston. On view are cards and photo reproductions of our city featuring main streets, churches, schools, parks, prisons, cemeteries, and five ...