perf: only allocate cacheableList once

src/Discord.Net.WebSocket/API/Gateway/MessageDeleteBulkEvent.cs

@@ -1,4 +1,4 @@
-#pragma warning disable CS1591
+#pragma warning disable CS1591
 using Newtonsoft.Json;
 using System.Collections.Generic;
 
@@ -9,6 +9,6 @@ namespace Discord.API.Gateway
         [JsonProperty("channel_id")]
         public ulong ChannelId { get; set; }
         [JsonProperty("ids")]
-        public IEnumerable<ulong> Ids { get; set; }
+        public ulong[] Ids { get; set; }
     }
 }

src/Discord.Net.WebSocket/DiscordSocketClient.cs

@@ -1375,13 +1375,13 @@ namespace Discord.WebSocket
                                             return;
                                         }
 
-                                        var cacheableList = ImmutableArray<Cacheable<IMessage, ulong>>.Empty;
+                                        var cacheableList = new List<Cacheable<IMessage, ulong>>(data.Ids.Length);
                                         foreach (ulong id in data.Ids)
                                         {
                                             var msg = SocketChannelHelper.RemoveMessage(channel, this, id);
                                             bool isCached = msg != null;
                                             var cacheable = new Cacheable<IMessage, ulong>(msg, id, isCached, async () => await channel.GetMessageAsync(id).ConfigureAwait(false));
-                                            cacheableList = cacheableList.Add(cacheable);
+                                            cacheableList.Add(cacheable);
 
                                             if (!ExclusiveBulkDelete ?? false) // this shouldn't happen, but we'll play it safe anyways
                                                 await TimedInvokeAsync(_messageDeletedEvent, nameof(MessageDeleted), cacheable, channel).ConfigureAwait(false);