Added GetUsers extensions to WebSocket->Channel

Cached Channels keep a local cache of the members of that channel. This commit adds a synchronus method to access the cached user stores.

It also fixes a bug where the GetUser extension would return an IGroupUser by soft-casting to IGroupUser, which would always return null. ISocketUser does not share an inheritance with IGroupUser, so I now return IUser instead.

src/Discord.Net/WebSocket/Extensions/ChannelExtensions.cs

@@ -1,4 +1,5 @@
 using System;
+using System.Collections.Generic;
 
 namespace Discord.WebSocket.Extensions
 {
@@ -7,15 +8,27 @@ namespace Discord.WebSocket.Extensions
         public static IUser GetUser(this IDMChannel channel, ulong id)
             => GetSocketDMChannel(channel).GetUser(id);
 
-        public static IGroupUser GetUser(this IGroupChannel channel, ulong id)
-            => GetSocketGroupChannel(channel).GetUser(id) as IGroupUser;
+        public static IEnumerable<IUser> GetUsers(this IDMChannel channel)
+            => GetSocketDMChannel(channel).Users;
+
+        public static IUser GetUser(this IGroupChannel channel, ulong id)
+            => GetSocketGroupChannel(channel).GetUser(id);
+
+        public static IEnumerable<IUser> GetUsers(this IGroupChannel channel)
+            => GetSocketGroupChannel(channel).Users;
 
         public static IGuildUser GetUser(this ITextChannel channel, ulong id)
             => GetSocketTextChannel(channel).GetUser(id);
 
+        public static IEnumerable<IGuildUser> GetUsers(this ITextChannel channel)
+            => GetSocketTextChannel(channel).Members;
+
         public static IGuildUser GetUser(this IVoiceChannel channel, ulong id)
             => GetSocketVoiceChannel(channel).GetUser(id);
 
+        public static IEnumerable<IGuildUser> GetUsers(this IVoiceChannel channel)
+            => GetSocketVoiceChannel(channel).Members;
+
         internal static SocketDMChannel GetSocketDMChannel(IDMChannel channel)
         {
             var socketChannel = channel as SocketDMChannel;