Add benchmark for hashing extensions

osu.Framework.Benchmarks/BenchmarkHashing.cs

···
+// Copyright (c) ppy Pty Ltd <[e-mail]>. Licensed under the MIT Licence.
+// See the LICENCE file in the repository root for full licence text.
+
+using System;
+using System.IO;
+using BenchmarkDotNet.Attributes;
+using osu.Framework.Extensions;
+
+namespace osu.Framework.Benchmarks
+{
+    [MemoryDiagnoser]
+    public class BenchmarkHashing
+    {
+        private const string test_string = @"A string with reasonable length";
+        private MemoryStream memoryStream;
+
+        [Benchmark]
+        public string StringMD5() => test_string.ComputeMD5Hash();
+
+        [Benchmark]
+        public string StringSHA() => test_string.ComputeSHA2Hash();
+
+        [GlobalSetup]
+        public void GlobalSetup()
+        {
+            byte[] array = new byte[1024];
+            var random = new Random(42);
+            random.NextBytes(array);
+            memoryStream = new MemoryStream(array);
+        }
+
+        [Benchmark]
+        public string StreamMD5() => memoryStream.ComputeMD5Hash();
+
+        [Benchmark]
+        public string StreamSHA() => memoryStream.ComputeSHA2Hash();
+    }
+}