One of the more distinctive buildings in Boston, at the base of Beacon Hill, right by the edge of the Charles River, is one whose very walls betray their original purpose: immense piles of very heavy stone, with thin slits for windows which barely let in the light. You don't have to look twice to know this is a prison. Or was --- the actual housing of prisoners moved some years ago to a newer facility, a few hundred yards away, with the general look from the outside, of a cubicle-infested office block. As to the older building, there is now a huge banner up proclaiming the way it's been repurposed. The old Charles Street Jail will soon be known as the Liberty Hotel. Call it a sign of the times.