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

initially it comes from https://github.com/php/php-src/commit/7c0e7b4fe8c22ba285e08874f2cc4c0702438d61#diff-89fb8b368c48bf62a149e0c2853141e7

and later on was moved to this location
https://github.com/php/php-src/commit/914cf3c0f89d0c1a9732c977ac58dd17868822b5#diff-358b07727650bd64ec27ffb307a91f2f