Revert "Use .NET public API to spawn the async method execution"

This reverts commit 076c105.

Author: sakno

src/DotNext/Runtime/CompilerServices/SpawningTaskMethodBuilder.cs

@@ -1,5 +1,6 @@
 using System.Runtime.CompilerServices;
 using System.Runtime.InteropServices;
+using System.Threading.Tasks.Sources;
 
 namespace DotNext.Runtime.CompilerServices;
 
@@ -31,8 +32,9 @@ public struct SpawningAsyncTaskMethodBuilder<TResult>()
     public void Start<TStateMachine>(ref TStateMachine stateMachine)
         where TStateMachine : IAsyncStateMachine
     {
-        var awaiter = System.Threading.Tasks.Task.Yield().GetAwaiter();
+        var awaiter = SpawningAsyncTaskMethodBuilder.Awaiter;
         builder.AwaitUnsafeOnCompleted(ref awaiter, ref stateMachine);
+        SpawningAsyncTaskMethodBuilder.Schedule();
     }
 
     /// <summary>
@@ -98,6 +100,11 @@ public void SetException(Exception e)
 [StructLayout(LayoutKind.Auto)]
 public struct SpawningAsyncTaskMethodBuilder()
 {
+    internal static readonly ConfiguredValueTaskAwaitable.ConfiguredValueTaskAwaiter Awaiter = new ValueTask(new SpecialCallbackDetector(), 0).ConfigureAwait(false).GetAwaiter();
+
+    [ThreadStatic]
+    private static (Action<object?> WorkItem, object? State) capturedArgs;
+
     private AsyncTaskMethodBuilder builder = AsyncTaskMethodBuilder.Create();
 
     /// <summary>
@@ -114,8 +121,9 @@ public struct SpawningAsyncTaskMethodBuilder()
     public void Start<TStateMachine>(ref TStateMachine stateMachine)
         where TStateMachine : IAsyncStateMachine
     {
-        var awaiter = Task.Yield().GetAwaiter();
+        var awaiter = Awaiter;
         builder.AwaitUnsafeOnCompleted(ref awaiter, ref stateMachine);
+        Schedule();
     }
 
     /// <summary>
@@ -167,4 +175,23 @@ public void SetException(Exception e)
     /// Gets the task representing the builder's asynchronous operation.
     /// </summary>
     public Task Task => builder.Task;
+
+    internal static void Schedule()
+    {
+        var (continuation, state) = capturedArgs;
+        capturedArgs = default;
+        ThreadPool.UnsafeQueueUserWorkItem(continuation, state, preferLocal: false);
+    }
+
+    private sealed class SpecialCallbackDetector : IValueTaskSource
+    {
+        void IValueTaskSource.GetResult(short token)
+        {
+        }
+
+        ValueTaskSourceStatus IValueTaskSource.GetStatus(short token) => ValueTaskSourceStatus.Pending;
+
+        void IValueTaskSource.OnCompleted(Action<object?> continuation, object? state, short token, ValueTaskSourceOnCompletedFlags flags)
+            => capturedArgs = (continuation, state);
+    }
 }