[RFC] Optional interfaces

[tontonsb]

This is an implementation attempt of https://wiki.php.net/rfc/optional-interfaces

It's my first attempt at modifying PHP sources, so I am very open to any suggestions and guidance on how this should be actually solved. But for now I'll briefly explain my approach:

• Types interface_name_list and interface_name are added to parser. interface_name_list is used instead of class_name_list for implements_list and interface_extends_list. The interface_name is a name that can be prefixed by a ? in which case it will set a ZEND_CLASS_NAME_OPTIONAL flag on the attr of that token.
• The struct zend_interface_name is defined and used for interface_names. The difference from zend_class_name is that the zend_interface_name also holds a boolean is_optional which is set according to the ZEND_CLASS_NAME_OPTIONAL flag.
• The inheritance mechanism uses the ZEND_FETCH_CLASS_SILENT flag on zend_fetch_class_by_name if the interface name is_optional. In case an optional interface fails to fetch, it's just skipped.

I'm not really sure what should be done for Opcache here and I haven't yet understood how to work with booleans there. I haven't looked into Reflection at all yet.

[nielsdos]

As you're asking for opcache help, this should resolve some questions and fix the tests (hopefully)