Store line item information in the bundle array via hook_entity_info_alter()

hook_commerce_line_item_info() allows line item types to be defined with several properties (including a variety of callbacks). Instead of keeping the line item info in its own data array, it would be good for all the info to be put in the bundle array on the entity definition. This would require changing the hook_entity_info() implementation in the line item module and the API functions for getting line item type information.

At the same time, it would be good for the callback information to be handled the same as callbacks for checkout panes and payment methods. This would involve having a single callbacks array on the bundle and using a callback naming pattern that would work by default for line item types without requiring the callbacks to be explicitly defined.