Call it a fun house. High above Lower Manhattan, a Tribeca penthouse that swaps elevators for a far more playful ride is searching for its next owner. The sprawling residence atop the landmarked building at 150 Nassau St. has hit the market for $20 million, according to an eye-catching StreetEasy update, and its most attention-grabbing feature isn’t the sweeping skyline views or the size of the apartment. Instead, it’s a gleaming four-story stainless-steel slide that spirals through the center