bitboard: make directional methods take step argument

src/repr/board.rs

@@ -298,7 +298,7 @@ impl Board {
 
             // Expand in direction until edge or non-opponent piece
             loop {
-                current = dir(&current);
+                current = dir(&current, 1);
                 if current.is_empty() || !current.intersects(*opponent_board) {
                     break;
                 }