[board] Implement piece sight algorithms

Add a new Sight trait, implemented by PlacedPiece. The implementation of this
trait produces a BitBoard representing the squares visible to the placed piece.

board/src/position/position.rs

@@ -44,6 +44,8 @@ pub struct Position {
 
     /// Bitboards representing positions of particular piece types per color.
     pieces_per_type: [[BitBoard; 6]; 2],
+
+    en_passant_square: Option<Square>,
 }
 
 impl Position {
@@ -70,6 +72,7 @@ impl Position {
                     BitBoard::empty(),
                 ],
             ],
+            en_passant_square: None,
         }
     }
 
@@ -101,6 +104,7 @@ impl Position {
                 black_pieces.iter().fold(BitBoard::empty(), |a, b| a | *b),
             ],
             pieces_per_type: [white_pieces, black_pieces],
+            en_passant_square: None,
         }
     }
 
@@ -155,6 +159,16 @@ impl Position {
         self.pieces_per_color[Color::White as usize] | self.pieces_per_color[Color::Black as usize]
     }
 
+    #[inline]
+    pub(crate) fn friendly_pieces(&self) -> BitBoard {
+        self.bitboard_for_color(self.color_to_move)
+    }
+
+    #[inline]
+    pub(crate) fn opposing_pieces(&self) -> BitBoard {
+        self.bitboard_for_color(self.color_to_move.other())
+    }
+
     /// Return a BitBoard representing the set of squares containing a piece.
     /// This set is the inverse of `occupied_squares`.
     #[inline]
@@ -195,6 +209,10 @@ impl Position {
     pub fn pieces(&self, color: Color) -> Pieces {
         Pieces::new(&self, color)
     }
+
+    pub fn en_passant_square(&self) -> Option<Square> {
+        self.en_passant_square
+    }
 }
 
 impl Default for Position {