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

DiscImageChef.Checksums/RIPEMD160Context.cs

@@ -30,6 +30,8 @@
 // Copyright © 2011-2018 Natalia Portillo
 // ****************************************************************************/
 
+#if !NETSTANDARD2_0
+
 using System.IO;
 using System.Security.Cryptography;
 using System.Text;
@@ -152,4 +154,6 @@ namespace DiscImageChef.Checksums
             return Data(data, (uint)data.Length, out hash);
         }
     }
-}
+}
+
+#endif