Add built-in support for closure callbacks for export PPP callbacks

[jamcleod]

Extends #21

Currently in order to use PPP closure callbacks for callbacks exported from the same plugin you'd need to import your own plugin. This is obviously not ideal and shouldn't be too bad to implement in a manner which uses the same API as the currently experimental <callback>::add_callback(...) function which the export_ppp_callback macro currently creates.

This is likely more ideal than trying to have it conform to the PppCallback API