Add toJSON methods (#1859)

* tojson things

* fix client

* ignore private properties

* remove extra property descriptors

* handle primitive flattening

* remove unused import

* add toJSON to collections

* reduce stateful props

* state

* allow custom prop names when flattening

* fix client

* fix build

* fix flatten docs

* remove guild.available, cleanup permissions, remove arbitrary id reduction

* fix util import

* add valueOf as needed, update member props

* fix incorrect merge

* update permissionoverwrites and permissions

remove serialization of permissions in PermissionOverwrites#toJSON.
change Permissions#toJSON to serialize permissions, by default excluding
admin checks.

* change Permissions#toJSON to return the primitive

* Permissions#toJSON explicitly return bitfield

src/structures/MessageMentions.js

@@ -1,4 +1,5 @@
 const Collection = require('../util/Collection');
+const Util = require('../util/Util');
 const GuildMember = require('./GuildMember');
 
 /**
@@ -139,6 +140,13 @@ class MessageMentions {
 
     return false;
   }
+
+  toJSON() {
+    return Util.flatten(this, {
+      members: true,
+      channels: true,
+    });
+  }
 }
 
 /**