Fix DDC-2802

Enables specifying MySQL index size on modifying (critical for `TEXT/BLOB` foreign keys):

~~~php
$index = new \Doctrine\DBAL\Schema\Index('my_idx', array('user_name(12)', 'last_login(34)'));
~~~

this will produce

~~~SQL
CREATE INDEX my_idx ON mytable (user_name(12), last_login(34));
~~~