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/util/Collection.js

@@ -1,3 +1,5 @@
+const Util = require('./Util');
+
 /**
  * A Map with additional utility methods. This is used throughout discord.js rather than Arrays for anything that has
  * an ID, for significantly improved performance and ease-of-use.
@@ -425,6 +427,10 @@ class Collection extends Map {
   sort(compareFunction = (x, y) => +(x > y) || +(x === y) - 1) {
     return new Collection([...this.entries()].sort((a, b) => compareFunction(a[1], b[1], a[0], b[0])));
   }
+
+  toJSON() {
+    return this.map(e => typeof e.toJSON === 'function' ? e.toJSON() : Util.flatten(e));
+  }
 }
 
 module.exports = Collection;