[board] Extend the Move type with two builder-style methods for capturing and promotion

These two methods replace self with one that has a populated capture or promotion field.

board/src/moves/move.rs

@@ -1,16 +1,35 @@
 // Eryn Wells <eryn@erynwells.me>
 
-use crate::piece::Piece;
+use crate::piece::{Piece, PlacedPiece, Shape};
 use crate::Square;
 
+#[derive(Debug, Clone, Eq, Hash, PartialEq)]
 pub struct Move {
-    pub piece: Piece,
-    pub from: Square,
-    pub to: Square,
+    piece: Piece,
+    from: Square,
+    to: Square,
+    capturing: Option<PlacedPiece>,
+    promoting_to: Option<Shape>,
 }
 
 impl Move {
     pub fn new(piece: Piece, from: Square, to: Square) -> Move {
-        Move { piece, from, to }
+        Move {
+            piece,
+            from,
+            to,
+            capturing: None,
+            promoting_to: None,
+        }
+    }
+
+    pub fn capturing(mut self, piece: PlacedPiece) -> Move {
+        self.capturing = Some(piece);
+        self
+    }
+
+    pub fn promoting_to(mut self, shape: Shape) -> Move {
+        self.promoting_to = Some(shape);
+        self
     }
 }