Extract the inference tree into a data structure.

When running suslik <example.syn> -r true, a trace is shown.

The successful subderivations of the synthesiser correspond to the trees. It would be good to have this tree captured as a byproduct of the synthesis, so it could be independently rendered in a format that can be checked by a third-party tool, such as HIP/SLEEK.

This issue is about the design of the derivation tree data type and a serialisation format for it.