Bump cart_id in uc_cart_products up from varchar(32)

As mentioned here: http://www.ubercart.org/forum/development/5111/cart_id_varchar
Session ids can be higher than 32 characters so cart_id should at minimum match the sessions table's sid of varchar(64) and possibly use varchar(255) to add the ability of using a prefix or suffix along with the session id to create multiple carts for users, as is done with http://drupal.org/project/uc_save_for_later