[board] Implement danger squares for the current player

This concept comes from [1]. Danger squares are the squares a king cannot move to
because it would permit the opposing player to capture the king on their next
turn.

[1]: https://peterellisjones.com/posts/generating-legal-chess-moves-efficiently/

core/src/pieces.rs

@@ -14,17 +14,17 @@ pub enum Shape {
 }
 
 impl Shape {
-    pub fn iter() -> Iter<'static, Shape> {
-        const ALL_SHAPES: [Shape; 6] = [
-            Shape::Pawn,
-            Shape::Knight,
-            Shape::Bishop,
-            Shape::Rook,
-            Shape::Queen,
-            Shape::King,
-        ];
+    pub const ALL: [Shape; 6] = [
+        Shape::Pawn,
+        Shape::Knight,
+        Shape::Bishop,
+        Shape::Rook,
+        Shape::Queen,
+        Shape::King,
+    ];
 
-        ALL_SHAPES.iter()
+    pub fn iter() -> Iter<'static, Shape> {
+        Shape::ALL.iter()
     }
 
     pub fn promotable() -> Iter<'static, Shape> {