RecyclableLongList.IndexOf stability & performance improvements.

Recyclable.Collections/Parallel/SpinLockSlimmer.cs

@@ -0,0 +1,26 @@
+namespace Recyclable.Collections.Parallel
+{
+	public struct SpinLockSlimmer
+	{
+		private volatile int _locked;
+
+		public void Enter()
+		{
+			if (Interlocked.CompareExchange(ref _locked, 1, 0) == 0)
+			{
+				return;
+			}
+
+			SpinWait waiter = new();
+			while (Interlocked.CompareExchange(ref _locked, 1, 0) == 1)
+			{
+				waiter.SpinOnce();
+			}
+		}
+
+		public void Exit()
+		{
+			_locked = 0;
+		}
+	}
+}