This commit partially fixes#5, however it still needs some work. TODO:
* remove debugging printf calls,
* some tuning, to consume less memory,
* implement similar solution for interfaces
However it works on ph7_class and thus passes whole class into the VM, what causes memory overhead,
as finally we have to find this class on the VM's stack. Instead, we could pass some ph7_class_info
structure containing a name of class to look for and information about its inheritances.
