Zend Engine 3

I think it's worth considering bumping the version number for ng. Changes may be yet to come, sure, but phpng was such a big overhaul I think it's worth calling "Zend Engine 3".

I haven't updated extensions which check for the constant, but here's a list from grep:

    oa-res-27-90:php-src ajf$ grep -r ZEND_ENGINE_2 .
    Binary file ./.git/objects/pack/pack-779abadf31d95ebd50611dbd5e7f4670f60dd109.pack matches
    ./ext/mysql/php_mysql.c:#ifdef ZEND_ENGINE_2
    ./ext/mysql/php_mysql.c:#ifdef ZEND_ENGINE_2
    ./ext/mysql/php_mysql.c:#ifdef ZEND_ENGINE_2
    ./ext/oci8/oci8.c:#ifdef ZEND_ENGINE_2
    ./ext/oci8/oci8.c:#else /* ZEND_ENGINE_2 */
    ./ext/oci8/oci8.c:#endif /* ZEND_ENGINE_2 */
    ./ext/pdo_firebird/firebird_driver.c:#ifdef ZEND_ENGINE_2
    ./ext/xml/xml.c:/* #ifdef ZEND_ENGINE_2
    ./ext/xmlwriter/php_xmlwriter.h:#ifndef ZEND_ENGINE_2
    ./ext/zip/zip_stream.c:#ifdef ZEND_ENGINE_2
    ./ext/zip/zip_stream.c:#endif /* ZEND_ENGINE_2 */
    ./main/internal_functions_win32.c:#ifndef ZEND_ENGINE_2

Hmm, actually, I don't think I'll keep `ZEND_ENGINE_2` around. Code that checks for it should probably just be removed, since that's basically just checking for PHP 5, and it's been a decade since its release. Plus, this'd complicate checks by cross-5/7-compatible code, as `ZEND_ENGINE_2` wouldn't guarantee it's actually ZE2, you'd have to do `defined(ZEND_ENGINE_2) && !defined(ZEND_ENGINE_3)`, or worse, `!defined(ZEND_ENGINE_3)` which isn't future-proof against a hyptothetical Zend Engine 4.