misc: MutualGuilds optimization (#1545)

* Check Dictionary

Check Dictionary instead of creating a new IReadOnlyCollection and looping in it

* Add Remark to MutualGuilds

src/Discord.Net.WebSocket/Entities/Users/SocketUser.cs

@@ -44,8 +44,11 @@ namespace Discord.WebSocket
         /// <summary>
         ///     Gets mutual guilds shared with this user.
         /// </summary>
+        /// <remarks>
+        ///     This property will only include guilds in the same <see cref="DiscordSocketClient"/>.
+        /// </remarks>
         public IReadOnlyCollection<SocketGuild> MutualGuilds
-            => Discord.Guilds.Where(g => g.Users.Any(u => u.Id == Id)).ToImmutableArray();
+            => Discord.Guilds.Where(g => g.GetUser(Id) != null).ToImmutableArray();
 
         internal SocketUser(DiscordSocketClient discord, ulong id)
             : base(discord, id)