Yeah, calling class_alias in userland is not convenient.
But there is two problem, if we want to do auto aliasing.

1. side effect is there without vendor "Zend" namespace. 
2. We should arrangements "JsonSerializable" autoloading, before user loading implemented class.
```
<?php
class Foo implements JsonSerializable{} // will be "Fatal error: Interface 'JsonSerializable' not found"
Zend\Json\Encoder::encode();
```