[bitboard, board] Make BitBoard::EMPTY and BitBoard::FULL public

Deprecate the methods.

I think I'm undoing a change I made earlier. 🙃

board/src/movement.rs

@@ -13,7 +13,7 @@ impl Board {
         if let Some(piece) = self.get_piece(square) {
             piece.movement(square, self)
         } else {
-            BitBoard::empty()
+            BitBoard::EMPTY
         }
     }
 }

board/src/sight.rs

@@ -27,7 +27,7 @@ impl Board {
         if let Some(piece) = self.get_piece(square) {
             piece.sight(square, self)
         } else {
-            BitBoard::empty()
+            BitBoard::EMPTY
         }
     }
 
@@ -41,7 +41,7 @@ impl Board {
         self.friendly_occupancy(color)
             .occupied_squares(&IterationDirection::default())
             .map(|square| self.sight(square))
-            .fold(BitBoard::empty(), BitOr::bitor)
+            .fold(BitBoard::EMPTY, BitOr::bitor)
     }
 
     pub fn active_color_opposing_sight(&self) -> BitBoard {
@@ -60,7 +60,7 @@ impl Board {
                     Some(self.friendly_sight(c))
                 }
             })
-            .fold(BitBoard::empty(), BitOr::bitor)
+            .fold(BitBoard::EMPTY, BitOr::bitor)
     }
 }