refactor: store AsyncAF instance data in a WeakMap for privacy

BREAKING CHANGE: the inner promise within AsyncAF instances can no longer be accessed at, e.g., AsyncAF().data

lib/classes/AsyncAfWrapper.js

@@ -1,18 +1,20 @@
 import createNewlessClass from '../methods/_internal/createNewlessClass';
 import use from '../methods/other/use';
 
+const dataStore = new WeakMap();
+
 class AsyncAfWrapperProto {
   constructor(data) {
-    this.data = Promise[Array.isArray(data) ? 'all' : 'resolve'](data);
+    dataStore.set(this, Promise[Array.isArray(data) ? 'all' : 'resolve'](data));
   }
   then(resolve, reject) {
-    return new (this.constructor)(this.data.then(resolve, reject));
+    return new (this.constructor)(dataStore.get(this).then(resolve, reject));
   }
   catch(reject) {
     return this.then(null, reject);
   }
   finally(onFinally) {
-    return this.data.finally(onFinally);
+    return dataStore.get(this).finally(onFinally);
   }
 }