Yay! Faneuil Hall opens its doors to the public today after a 20-month, $6 million renovation. What's changed? Everything! And nothing! The main bit's still a public market, per Peter Faneuil's original 1742 stipulations, and no one touched the Grand Hall, mostly because no one needed to. No, the biggest changes were electrical and lighting upgrades; and the addition of a National Park Service visitors’ center that includes an interactive information booth. There's also a classroom in the basement now, so the service can lecture. Finally, that public market space does have a new look, what one vendor described to The Globe as "rustic industrial."