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

Since a lot of code may do #ifdef ZEND_ENGINE_2, maybe it'd be good to leave ZEND_ENGINE_2 there but also add ZEND_ENGINE_3 in addition to it?