For generators ZEND_RETURN directly calls ZEND_VM_RETURN(), thus passing
execution back to the caller (zend_generator_resume).
This commit also adds a check that only return; is used in generators and
not return $value;.
This adds another function execute_ex(), which accepts a zend_execute_data
struct to run (contrary to execute(), which accepts a zend_op_array from
which it initialized the execute_data).
This needs a bit more cleanup.
The Generator class now uses a zend_generator struct, so it'll be able to
store additional info.
This commit also ensures that Generator cannot be directly instantiated
and extended. The error tests are now in a separate folder from the
(yet-to-come) functional tests.
