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/util/ActivityFlags.js

@@ -0,0 +1,29 @@
+const BitField = require('./BitField');
+
+/**
+ * Data structure that makes it easy to interact with an {@link Activity#flags} bitfield.
+ * @extends {BitField}
+ */
+class ActivityFlags extends BitField {}
+
+/**
+ * Numeric activity flags. All available properties:
+ * * `INSTANCE`
+ * * `JOIN`
+ * * `SPECTATE`
+ * * `JOIN_REQUEST`
+ * * `SYNC`
+ * * `PLAY`
+ * @type {Object}
+ * @see {@link https://discordapp.com/developers/docs/topics/gateway#activity-object-activity-flags}
+ */
+ActivityFlags.FLAGS = {
+  INSTANCE: 1 << 0,
+  JOIN: 1 << 1,
+  SPECTATE: 1 << 2,
+  JOIN_REQUEST: 1 << 3,
+  SYNC: 1 << 4,
+  PLAY: 1 << 5,
+};
+
+module.exports = ActivityFlags;