Add various optimizations and cleanups (#1114)

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

* Add changes according to 194a8aa427

src/Discord.Net.WebSocket/Entities/Channels/SocketDMChannel.cs

@@ -97,7 +97,7 @@ namespace Discord.WebSocket
             if (id == Recipient.Id)
                 return Recipient;
             else if (id == Discord.CurrentUser.Id)
-                return Discord.CurrentUser as SocketSelfUser;
+                return Discord.CurrentUser;
             else
                 return null;
         }

src/Discord.Net.WebSocket/Entities/Channels/SocketGroupChannel.cs

@@ -18,10 +18,10 @@ namespace Discord.WebSocket
     public class SocketGroupChannel : SocketChannel, IGroupChannel, ISocketPrivateChannel, ISocketMessageChannel, ISocketAudioChannel
     {
         private readonly MessageCache _messages;
+        private readonly ConcurrentDictionary<ulong, SocketVoiceState> _voiceStates;
 
         private string _iconId;
         private ConcurrentDictionary<ulong, SocketGroupUser> _users;
-        private ConcurrentDictionary<ulong, SocketVoiceState> _voiceStates;
 
         public string Name { get; private set; }
 
@@ -129,7 +129,7 @@ namespace Discord.WebSocket
         internal SocketGroupUser GetOrAddUser(UserModel model)
         {
             if (_users.TryGetValue(model.Id, out SocketGroupUser user))
-                return user as SocketGroupUser;
+                return user;
             else
             {
                 var privateUser = SocketGroupUser.Create(this, Discord.State, model);
@@ -143,7 +143,7 @@ namespace Discord.WebSocket
             if (_users.TryRemove(id, out SocketGroupUser user))
             {
                 user.GlobalUser.RemoveRef(Discord);
-                return user as SocketGroupUser;
+                return user;
             }
             return null;
         }