[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.

position/src/lib.rs

@@ -14,4 +14,5 @@ mod macros;
 #[macro_use]
 mod testing;
 
-pub use position::{MakeMoveError, MoveBuilder as MakeMoveBuilder, Position};
+pub use chessfriend_board::{fen, PlacePieceError, PlacePieceStrategy};
+pub use position::{CastleEvaluationError, Position, ValidateMove};