Refactor thread pool implementation.

- Make mjTask non-opaque and remove C++ Task class.
- Make C++ thread pool a subclass of a skeletal mjThreadPool C struct.
- Make the mjTask status enum more consistent with the rest of MuJoCo.
- Change mju_threadPoolEnqueue to take just the mjTask. Users must now prepare the mjTask by assigning the function pointer and argument into the struct.
- Rename files in thread/ to be more consistent with the rest of MuJoCo.
- Run threading tests in CMake.
- Allow use of C++20 designated initializers.

Otherwise the functionality remains identical.

PiperOrigin-RevId: 564374675
Change-Id: I37c9894566bc39faf217e5aa97a4e2713a70e467

src/thread/thread_task.h

@@ -0,0 +1,50 @@
+// Copyright 2023 DeepMind Technologies Limited
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#ifndef MUJOCO_SRC_THREAD_THREAD_TASK_H_
+#define MUJOCO_SRC_THREAD_THREAD_TASK_H_
+
+#include <atomic>
+#include <new>
+#include <type_traits>
+
+#include <mujoco/mjexport.h>
+#include <mujoco/mjthread.h>
+
+#ifdef __cplusplus
+namespace mujoco {
+extern "C" {
+#endif
+
+// Initialize an mjTask.
+MJAPI void mju_defaultTask(mjTask* task);
+
+// Wait for a task to complete.
+MJAPI void mju_taskJoin(mjTask* task);
+
+#ifdef __cplusplus
+}  // extern "C"
+
+using TaskStatus = std::remove_volatile_t<decltype(mjTask::status)>;
+inline std::atomic<TaskStatus>& GetAtomicTaskStatus(mjTask* task) {
+  static_assert(sizeof(std::atomic<TaskStatus>) == sizeof(TaskStatus));
+  static_assert(alignof(std::atomic<TaskStatus>) == alignof(TaskStatus));
+  static_assert(std::atomic<TaskStatus>::is_always_lock_free);
+  return *std::launder(reinterpret_cast<std::atomic<TaskStatus>*>(
+      const_cast<TaskStatus*>(&task->status)));
+}
+}  // namespace mujoco
+#endif  // __cplusplus
+
+#endif  // MUJOCO_SRC_THREAD_THREAD_TASK_H_