Add depth based time limit

src/search/iterative_deepening.rs

@@ -17,7 +17,7 @@ pub fn iterative_deepening(
     let mut best_move = None;
 
     for depth in 1..=max_depth {
-        if time.exceed_hard_limit() {
+        if time.exceed_soft_limit() {
             return Ok(best_move);
         }