Fix overflow bug, clean up code

src/search/move_ordering.rs

@@ -21,7 +21,7 @@ pub fn score_move(
     if mv.is_capture() {
         let aggressor = mailbox.piece_at(mv.src()).unwrap();
         if let Some(victim) = mailbox.piece_at(mv.dst()) {
-            return aggressor.0.idx() as i16 - 8 * victim.0.idx() as i16;
+            return aggressor.0.idx() as i32 - 8 * victim.0.idx() as i32;
         }
         return 0; //en passant
     }