[board] Remove BoardSide enum; use Castle everywhere

Move Castle to a castle module inside the move module.
Implement into_index() on Castle to turn it into a usize.

board/src/position/position.rs

@@ -11,23 +11,6 @@ use crate::{
 };
 use std::{cell::OnceCell, fmt};
 
-/// A lateral side of the board relative to the player. Kingside is the side the
-/// player's king starts on. Queenside is the side of the board the player's
-/// queen starts on.
-pub(crate) enum BoardSide {
-    King = 0,
-    Queen = 1,
-}
-
-impl From<Castle> for BoardSide {
-    fn from(value: Castle) -> Self {
-        match value {
-            Castle::KingSide => BoardSide::King,
-            Castle::QueenSide => BoardSide::Queen,
-        }
-    }
-}
-
 #[derive(Clone, Debug, Eq, PartialEq)]
 pub struct Position {
     color_to_move: Color,