[WIP] Add serializer interface

Targets #161.
The getter _and_ setters were added because of the idea of doing `ws_object_run()` on the object. The dereived classes do something, either transforming a transaction to a buffer or the other way round. So If you put a buffer into the backend, you can run it and get a transaction from it as well as the other way round.

That was my idea when writing this.

If I understand things right, you want me to implement an extended object type table which holds the callbacks for what exactly? You need _either_ to pass a buffer _or_ a transaction and get the other one out of the backend. How would you implement an interface for this?

(your patches to this are welcome)