* small code cleanup
* implement new structure for storing information about class inheritance
* implement PH7_NewClassInfo()
* make a use of smaller ph7_class_info instead of ph7_class, to reduce memory usage
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.
* do not allow two or more classes / interfaces with the same name
* fix class_exists() builtin function so that it does not return true if interface is found
