[board, core, moves, position] Implement castling

Implement a new method on Position that evaluates whether the active color can castle
on a given wing of the board. Then, implement making a castling move in the position.

Make a new Wing enum in the core crate to specify kingside or queenside. Replace the
Castle enum from the board crate with this one. This caused a lot of churn...

Along the way fix a bunch of tests.

Note: there's still no way to actually make a castling move in explorer.

core/src/lib.rs

@@ -8,6 +8,6 @@ pub mod shapes;
 mod macros;
 
 pub use colors::Color;
-pub use coordinates::{Direction, File, Rank, Square};
+pub use coordinates::{Direction, File, Rank, Square, Wing};
 pub use pieces::{Piece, PlacedPiece};
 pub use shapes::Shape;