[board] Implement Board::unwrap_color

Unwrap an Option<Color> using the board's active color in the default case.

board/src/board.rs

@@ -117,6 +117,13 @@ impl Board {
     }
 }
 
+impl Board {
+    #[must_use]
+    pub fn unwrap_color(&self, color: Option<Color>) -> Color {
+        color.unwrap_or(self.active_color)
+    }
+}
+
 #[cfg(test)]
 mod tests {
     use super::*;