[bitboard, core, position] Implement proper castle move generation

Add a field to MoveSet called special that flags special moves that should be
generated in the iter() method. This field is a u8. It only tracks castles in
the first and second bits (kingside and queenside, respectively). The move iterator
chains two maps over Option<()> that produce the kingside and queenside castle
moves.

With that done, finish the implementation of Position::player_can_castle by
adding checks for whether the squares between the rook and king are clear, and
that the king would not pass through a check. This is done with BitBoards!

Finally, implement some logic in PositionBuilder that updates the position's
castling flags based on the positions of king and rooks.

Supporting changes:
- Add Color:ALL and iterate on that slice
- Add Castle::ALL and iterator on that slice
- Add a CastlingParameters struct that contains BitBoard properties that describe
  squares that should be clear of pieces and squares that should not be attacked.

core/src/colors.rs

@@ -10,8 +10,10 @@ pub enum Color {
 }
 
 impl Color {
-    pub fn iter() -> impl Iterator<Item = Color> {
-        [Color::White, Color::Black].into_iter()
+    pub const ALL: [Color; 2] = [Color::White, Color::Black];
+
+    pub fn iter() -> impl Iterator<Item = &'static Color> {
+        Color::ALL.iter()
     }
 
     pub fn other(&self) -> Color {