Extend pretty printer for declarations

[Toxaris]

The current pretty printer only supports expressions, but we want to also print the program that results from transformations. Maybe @lordxist can do this?

[lordxist]

I'm almost finished with a pretty printer for program trees. It currently works with the Uroboro version from before your pull requests were accepted. (But this can be easily adapted to other versions.) I could create a repository for this and my other code (for transformations etc.), with Uroboro as a dependency.

[Toxaris]

Cool.