[bitboard] Implement a Square type; fix all the build issues after adding a mod line to lib.rs

bitboard/src/lib.rs

@@ -1,10 +1,3 @@
-#[cfg(test)]
-mod tests {
-    use super::*;
-
-    #[test]
-    fn it_works() {
-        let result = add(2, 2);
-        assert_eq!(result, 4);
-    }
-}
+mod bitboard;
+mod position;
+mod square;