Use neighbours method in predator and prey.

src/bin/predprey/main.rs

@@ -71,8 +71,7 @@ fn send<T: Write>(w: &mut T, board: &Board) -> std::io::Result<()> {
 const DEFAULT_ARG: usize = 10;
 
 fn grow(board: &Board, pos: Point2d, grow: Square, die: Square) -> Square {
-    for dir in Vec2d::directions() {
-        let pos1 = pos + dir;
+    for pos1 in pos.neighbours() {
         if board.rect().contains(pos1) {
             let neigh_sq = board.get(pos1);
             if neigh_sq == grow {