[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/coordinates/wings.rs

@@ -0,0 +1,22 @@
+// Eryn Wells <eryn@erynwells.me>
+
+#[repr(u8)]
+#[derive(Clone, Copy, Debug, Eq, PartialEq)]
+pub enum Wing {
+    KingSide = 0,
+    QueenSide = 1,
+}
+
+impl Wing {
+    pub const NUM: usize = 2;
+    pub const ALL: [Wing; Self::NUM] = [Self::KingSide, Self::QueenSide];
+}
+
+impl std::fmt::Display for Wing {
+    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
+        match self {
+            Wing::KingSide => write!(f, "kingside"),
+            Wing::QueenSide => write!(f, "queenside"),
+        }
+    }
+}