refactor(BitField): base class for Permissions, ActivityFlags, Speaking (#2765)

* abstract BitField from Permissions

* reduce useless code, improve docs

* add a ReadOnly identifier to the return type of Bitfield#freeze()

https://www.typescriptlang.org/docs/handbook/release-notes/typescript-2-1.html#partial-readonly-record-and-pick

* fix the RangeError

* update docs, convert Speaking and ActivityFlags to bitfields

* fix some docs

* Fix Speaking BitField oops

* docs for oops

* more incorrect docs

* Fix incorrectly named property

* add new classes to index

* fix missing @extends docs

* default bitfield resolve to 0, and cleanup defaulting everywhere

Also removes GuildMember#missiongPermissions() alias that had incorrect behavior

* Breaking: Rename Overwrite allowed and denied to allow and deny

To be consistent with the api's naming

* fix setSpeaking usage to bitfields instead of booleans

* fix speaking bug in playChunk

* docs: Updated typings

* fix: BitFieldResolvable should use RecursiveArray

* bugfix/requested change

* typings: Cleanup (#2)

* typings: Fix BitField#{toArray,@@iterator} output type

* typings: correct PermissionOverwrites property names and nitpicks

src/structures/Presence.js

@@ -1,5 +1,6 @@
 const Util = require('../util/Util');
-const { ActivityTypes, ActivityFlags } = require('../util/Constants');
+const ActivityFlags = require('../util/ActivityFlags');
+const { ActivityTypes } = require('../util/Constants');
 
 /**
  * Activity sent in a message.
@@ -150,15 +151,12 @@ class Activity {
     this.assets = data.assets ? new RichPresenceAssets(this, data.assets) : null;
 
     this.syncID = data.sync_id;
-    this._flags = data.flags;
-  }
 
-  get flags() {
-    const flags = [];
-    for (const [name, flag] of Object.entries(ActivityFlags)) {
-      if ((this._flags & flag) === flag) flags.push(name);
-    }
-    return flags;
+    /**
+     * Flags that describe the activity
+     * @type {ReadoOnly<ActivityFlags>}
+     */
+    this.flags = new ActivityFlags(data.flags).freeze();
   }
 
   /**