refactor: make hash dump usuable for debugging

hashtest/insert.tst.ref

@@ -1,34 +1,34 @@
 <hash elem=5>
-	0:0:
-	1:0:
-	2:0:
-	3:0:
-	4:0:
-	5:0:
-	6:0:
-	7:0:
-	8:0:
-	9:0:
+	0:<empty>
+	1:<empty>
+	2:<empty>
+	3:<empty>
+	4:<empty>
+	5:<empty>
+	6:<empty>
+	7:<empty>
+	8:<empty>
+	9:<empty>
 	10:10:aa
 	11:10:ab
 	12:10:ac
-	13:0:
-	14:0:
-	15:0:
-	16:0:
-	17:0:
-	18:0:
-	19:0:
+	13:<empty>
+	14:<empty>
+	15:<empty>
+	16:<empty>
+	17:<empty>
+	18:<empty>
+	19:<empty>
 	20:20:ba
 	21:21:bb
-	22:0:
-	23:0:
-	24:0:
-	25:0:
-	26:0:
-	27:0:
-	28:0:
-	29:0:
-	30:0:
-	31:0:
+	22:<empty>
+	23:<empty>
+	24:<empty>
+	25:<empty>
+	26:<empty>
+	27:<empty>
+	28:<empty>
+	29:<empty>
+	30:<empty>
+	31:<empty>
 </hash>