Pacific Atlantic Water Flow

Write `pacific_atlantic(heights)` returning every cell from which water can reach both oceans. Water flows to a neighbour of equal or lower height. The Pacific touches the top and left edges, the Atlantic the bottom and right. Searching downhill from every cell is O(cells^2). Instead run two searches *uphill* from the edges — from the ocean, climbing to cells of equal or greater height — and intersect the two reachable sets. Flipping the comparison is the entire trick.