introduce unifex::async_resource

- async_destroy CPO
- returns a ptr-like object
- sample tests

include/unifex/async_destroy.hpp

@@ -0,0 +1,36 @@
+// (c) Meta Platforms, Inc. and affiliates. Confidential and proprietary.
+
+#pragma once
+
+#include <unifex/just.hpp>
+#include <unifex/tag_invoke.hpp>
+
+namespace unifex {
+// meant for unifex upstream, different naming convention
+// @nolint(readability-identifier-naming)
+
+inline constexpr struct _destroy_fn {
+private:
+  template <typename Resource>
+  static auto try_invoke_member(Resource& resource, int) noexcept
+      -> decltype(resource.destroy()) {
+    return resource.destroy();
+  }
+
+  template <typename Resource>
+  [[deprecated("noop async_destroy")]] static auto
+  try_invoke_member(Resource&, double) noexcept {
+    return unifex::just();
+  }
+
+public:
+  template <typename Resource>
+  constexpr auto operator()(Resource& resource) const noexcept {
+    if constexpr (unifex::tag_invocable<_destroy_fn, Resource&>) {
+      return unifex::tag_invoke(_destroy_fn{}, resource);
+    } else {
+      return try_invoke_member(resource, 1);
+    }
+  }
+} async_destroy{};
+}  // namespace unifex