util/set: add a basic map-based Set type

We have two other types of Sets here. Add the basic obvious one too.

Needed for a change elsewhere.

Updates #cleanup

Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

util/set/set_test.go

@@ -0,0 +1,24 @@
+// Copyright (c) Tailscale Inc & AUTHORS
+// SPDX-License-Identifier: BSD-3-Clause
+
+package set
+
+import "testing"
+
+func TestSet(t *testing.T) {
+	s := Set[int]{}
+	s.Add(1)
+	s.Add(2)
+	if !s.Contains(1) {
+		t.Error("missing 1")
+	}
+	if !s.Contains(2) {
+		t.Error("missing 2")
+	}
+	if s.Contains(3) {
+		t.Error("shouldn't have 3")
+	}
+	if s.Len() != 2 {
+		t.Errorf("wrong len %d; want 2", s.Len())
+	}
+}