Very basic implementation of support for a "currency"-system. (Petra-AI complies.)
This should be extended to something like:
> Resource A can be bartered into [Resource B, Resource C].
> Resource B can be bartered into [Resource A, Resource D].
> Resource C can be bartered into [Resource D].
> Resource D can be bartered into [Resource A, Resource B, Resource C].
for maximum moddability.
For now it is mainly a reference for @wowgetoffyourcellphone and @Nescio.