util/cmpx: add package with cmp-like things from future Go releases

Updates #8296

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

util/cmpx/cmpx_test.go

@@ -0,0 +1,24 @@
+// Copyright (c) Tailscale Inc & AUTHORS
+// SPDX-License-Identifier: BSD-3-Clause
+
+package cmpx
+
+import "testing"
+
+func TestOr(t *testing.T) {
+	if g, w := Or[string](), ""; g != w {
+		t.Errorf("got %v; want %v", g, w)
+	}
+	if g, w := Or[int](), 0; g != w {
+		t.Errorf("got %v; want %v", g, w)
+	}
+	if g, w := Or("", "foo", "bar"), "foo"; g != w {
+		t.Errorf("got %v; want %v", g, w)
+	}
+	if g, w := Or("foo", "bar"), "foo"; g != w {
+		t.Errorf("got %v; want %v", g, w)
+	}
+	if g, w := Or("", "", "bar"), "bar"; g != w {
+		t.Errorf("got %v; want %v", g, w)
+	}
+}