zend_generator: Fix zend_object std layout for zend_generator (draft)(cannot-pass-tests)

[Snape3058]

Please refer to #17598 for more details.

This patch tries to fix the zend_object layout problem for zend_generator according to the requirements since PHP 7.

[dstogov]

See the test failures

[cmb69]

Before going into the "details" here, we first should decide if this layout change should be done at all. According to #17598 (comment):

I thought we had the zend_object on purpose as the first member on those classes which don't use properties.
The purpose being avoiding useless offset computations and better locality for the trivial refcounting which is always in the first place when zend_object is the first item.