syncs: add generic Pool (#12759)

Pool is a type-safe wrapper over sync.Pool.

Updates tailscale/corp#11038
Updates #cleanup

Signed-off-by: Joe Tsai <joetsai@digital-static.net>

syncs/pool_test.go

@@ -0,0 +1,30 @@
+// Copyright (c) Tailscale Inc & AUTHORS
+// SPDX-License-Identifier: BSD-3-Clause
+
+package syncs
+
+import "testing"
+
+func TestPool(t *testing.T) {
+	var pool Pool[string]
+	s := pool.Get() // should not panic
+	if s != "" {
+		t.Fatalf("got %q, want %q", s, "")
+	}
+	pool.New = func() string { return "new" }
+	s = pool.Get()
+	if s != "new" {
+		t.Fatalf("got %q, want %q", s, "new")
+	}
+	var found bool
+	for range 1000 {
+		pool.Put("something")
+		found = pool.Get() == "something"
+		if found {
+			break
+		}
+	}
+	if !found {
+		t.Fatalf("unable to get any value put in the pool")
+	}
+}