[WIP] Objects support

This adds very minimal objects supports

What works:

 - declaring a class
 - instanciating
 - calling methods
 - stdClass
 - casting to object

TODO:

 - get/set properties
 - default property values
 - $this
 - visibility, scope
 - extending
 - abstract methods, classes
 - interfaces
 - instanceof, get_class, ...
 - self, static, parent
 - magic methods
 - constructor
 - var_dump

Maybe all this could be done incrementally in separate PRs