Add various optimizations and cleanups (#1114)

* Change all Select(... as ...) to OfType

* Add changes according to 194a8aa427

src/Discord.Net.Core/Format.cs

@@ -1,9 +1,9 @@
-namespace Discord
+namespace Discord
 {
     public static class Format
     {
         // Characters which need escaping
-        private static string[] SensitiveCharacters = { "\\", "*", "_", "~", "`" };
+        private static readonly string[] SensitiveCharacters = { "\\", "*", "_", "~", "`" };
 
         /// <summary> Returns a markdown-formatted string with bold formatting. </summary>
         public static string Bold(string text) => $"**{text}**";