fixes hash(HashSet) which was wrong as it didn't respect tombstones; refs #13649

lib/pure/collections/sets.nim

@@ -574,7 +574,8 @@ proc map*[A, B](data: HashSet[A], op: proc (x: A): B {.closure.}): HashSet[B] =
 proc hash*[A](s: HashSet[A]): Hash =
   ## Hashing of HashSet.
   for h in 0 .. high(s.data):
-    result = result xor s.data[h].hcode
+    if isFilledAndValid(s.data[h].hcode):
+      result = result xor s.data[h].hcode
   result = !$result
 
 proc `$`*[A](s: HashSet[A]): string =