BenchmarkDotNet snippet to compare Span<byte> to raw pointer (#370)

* BenchmarkDotNet snippet to compare Span<byte> to raw pointer work.

* nits

* nit

benchmark/BDN.benchmark/SpanVsPointer.cs

@@ -0,0 +1,68 @@
+// Copyright (c) Microsoft Corporation.
+// Licensed under the MIT license.
+
+using System.Runtime.CompilerServices;
+using BenchmarkDotNet.Attributes;
+
+namespace BDN.benchmark
+{
+    public unsafe class SpanVsPointer
+    {
+        const int Count = 1024;
+
+        byte[] bytes;
+        byte* bytesPtr;
+
+        [GlobalSetup]
+        public void GlobalSetup()
+        {
+            bytes = GC.AllocateArray<byte>(Count, true);
+            bytesPtr = (byte*)Unsafe.AsPointer(ref bytes[0]);
+            for (var ii = 0; ii < Count; ++ii)
+                bytes[ii] = (byte)ii;
+        }
+
+        [BenchmarkCategory("Swap"), Benchmark(Baseline = true)]
+        public void Pointer()
+        {
+            byte* pointer = bytesPtr;
+            byte* end = pointer + Count - 1;
+            while (pointer < end)
+            {
+                var tmp = *pointer;
+                *pointer = *++pointer;
+                *pointer = tmp;
+            }
+        }
+
+        [BenchmarkCategory("Swap"), Benchmark]
+        public void Span()
+        {
+            var span = new Span<byte>(bytesPtr, Count);
+            int i = 0;
+            while (i < Count - 1)
+            {
+                var tmp = span[i];
+                span[i] = span[++i];
+                span[i] = tmp;
+            }
+        }
+
+        [BenchmarkCategory("Swap"), Benchmark]
+        public void SpanToPointer()
+        {
+            var span = new Span<byte>(bytesPtr, Count);
+            fixed (byte* ptr = span)
+            {
+                byte* pointer = ptr;
+                byte* end = pointer + Count - 1;
+                while (pointer < end)
+                {
+                    var tmp = *pointer;
+                    *pointer = *++pointer;
+                    *pointer = tmp;
+                }
+            }
+        }
+    }
+}