[board] Implement some castling checks on Move

Define some constants in Square that refer to the starting positions of the two
kings, and the target castling squares. Then implement the following methods that
use those constants to determine if a move is a castle.

- Move::is_kingside_castle()
- Move::is_queenside_castle()
- Move::is_castle()

These checks only apply to King moves, and if the king is moving from and to
specific squares.

board/src/moves/move.rs

@@ -1,7 +1,10 @@
 // Eryn Wells <eryn@erynwells.me>
 
-use crate::piece::{Piece, PlacedPiece, Shape};
-use crate::Square;
+use crate::{
+    piece::{Piece, PlacedPiece, Shape},
+    position::BoardSide,
+    Square,
+};
 
 #[derive(Debug, Clone, Eq, Hash, PartialEq)]
 pub struct Move {
@@ -32,4 +35,27 @@ impl Move {
         self.promoting_to = Some(shape);
         self
     }
+
+    pub fn is_castle(&self) -> bool {
+        let color = self.piece.color();
+        self.piece.shape() == Shape::King
+            && self.from == Square::KING_STARTING_SQUARES[color as usize]
+            && Square::KING_CASTLE_TARGET_SQUARES[color as usize].contains(&self.to)
+    }
+
+    pub fn is_kingside_castle(&self) -> bool {
+        let color = self.piece.color();
+        self.piece.shape() == Shape::King
+            && self.from == Square::KING_STARTING_SQUARES[color as usize]
+            && self.to
+                == Square::KING_CASTLE_TARGET_SQUARES[color as usize][BoardSide::King as usize]
+    }
+
+    pub fn is_queenside_castle(&self) -> bool {
+        let color = self.piece.color();
+        self.piece.shape() == Shape::King
+            && self.from == Square::KING_STARTING_SQUARES[color as usize]
+            && self.to
+                == Square::KING_CASTLE_TARGET_SQUARES[color as usize][BoardSide::Queen as usize]
+    }
 }

board/src/square.rs

@@ -155,6 +155,10 @@ impl Square {
 }
 
 impl Square {
+    pub(crate) const KING_STARTING_SQUARES: [Square; 2] = [Square::E1, Square::E8];
+    pub(crate) const KING_CASTLE_TARGET_SQUARES: [[Square; 2]; 2] =
+        [[Square::C1, Square::G1], [Square::C8, Square::G8]];
+
     pub fn from_algebraic_str(s: &str) -> Result<Square, ParseSquareError> {
         s.parse()
     }