util/set: add iterator support to Set[T] (#20159)

This patch adds:

- Set.All which returns an iter.Seq to complement Set.Slice.

- Set.AddSeq which adds an iter.Seq.

- Set.DeleteSeq which deletes an iter.Seq to complement Set.AddSeq
  and provide the missing method for deleting multiple elements.

- Set.DeleteSlice and Set.DeleteSet to complement AddSlice and AddSet.

Updates #cleanup

Signed-off-by: Simon Law <sfllaw@tailscale.com>

util/set/set_test.go

@@ -50,6 +50,40 @@ func TestSet(t *testing.T) {
 			t.Errorf("slice missing %d (%#v)", e, es)
 		}
 	}
+
+	s.Delete(1)
+	if s.Contains(1) {
+		t.Error("shouldn't have 1")
+	}
+	if !s.Contains(2) {
+		t.Error("missing 2")
+	}
+	if !s.Contains(3) {
+		t.Error("missing 3")
+	}
+	if !s.Contains(4) {
+		t.Error("missing 4")
+	}
+	if s.Len() != 3 {
+		t.Errorf("wrong len %d; want 3", s.Len())
+	}
+
+	s.DeleteSeq(slices.Values([]int{2, 3}))
+	if s.Contains(1) {
+		t.Error("shouldn't have 1")
+	}
+	if s.Contains(2) {
+		t.Error("shouldn't have 2")
+	}
+	if s.Contains(3) {
+		t.Error("shouldn't have 3")
+	}
+	if !s.Contains(4) {
+		t.Error("missing 4")
+	}
+	if s.Len() != 1 {
+		t.Errorf("wrong len %d; want 1", s.Len())
+	}
 }
 
 func TestSetOf(t *testing.T) {