Migrate DiscUtils fat code to a standalone project dependency

DiscUtils/DiscUtils.Core/CoreCompat/EncodingHelper.cs

@@ -0,0 +1,23 @@
+#if !NET40 && !NET45
+using System.Text;
+#endif
+
+namespace DiscUtils.CoreCompat
+{
+    public static class EncodingHelper
+    {
+        private static bool _registered;
+
+        public static void RegisterEncodings()
+        {
+            if (_registered)
+                return;
+
+            _registered = true;
+
+#if !NET40 && !NET45
+            Encoding.RegisterProvider(CodePagesEncodingProvider.Instance);
+#endif
+        }
+    }
+}