pw_thread: add helper ThreadCore interface

Also extends the pw_thread documentation regarding thread creation.

Change-Id: Ib0a0572060188244b9a5cf91a6bba575057f405b
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/41205
Reviewed-by: Keir Mierle <keir@google.com>
Reviewed-by: Wyatt Hepler <hepler@google.com>
Reviewed-by: Paul Mathieu <paulmathieu@google.com>
Pigweed-Auto-Submit: Ewout van Bekkum <ewout@google.com>
Commit-Queue: Auto-Submit <auto-submit@pigweed.google.com.iam.gserviceaccount.com>
diff --git a/pw_thread/thread.cc b/pw_thread/thread.cc
new file mode 100644
index 0000000..d2844e5
--- /dev/null
+++ b/pw_thread/thread.cc
@@ -0,0 +1,30 @@
+// Copyright 2021 The Pigweed Authors
+//
+// 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
+//
+//     https://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.
+
+#include "pw_thread/thread.h"
+
+namespace pw::thread {
+namespace {
+
+void StartThreadCore(void* void_thread_core_ptr) {
+  static_cast<ThreadCore*>(void_thread_core_ptr)->Start();
+}
+
+}  // namespace
+
+// Delegating constructor which defers to the facade's constructor.
+Thread::Thread(const Options& options, ThreadCore& thread_core)
+    : Thread(options, StartThreadCore, &thread_core) {}
+
+}  // namespace pw::thread