Move libraries to .NET Standard, allowing Xamarin.macOS GUI to compile.

DiscImageChef.Core/Benchmark.cs

@@ -361,6 +361,7 @@ namespace DiscImageChef.Core
             results.SeparateTime += (end - start).TotalSeconds;
             #endregion MD5
 
+#if !NETSTANDARD2_0
             #region RIPEMD160
             ctx = new Ripemd160Context();
             ms.Seek(0, SeekOrigin.Begin);
@@ -392,6 +393,7 @@ namespace DiscImageChef.Core
                                 });
             results.SeparateTime += (end - start).TotalSeconds;
             #endregion RIPEMD160
+#endif
 
             #region SHA1
             ctx = new Sha1Context();