Stricter testing of the prettyprinter/parser roundtrip

[hanjoosten]

Recently I have been trying to do some initial work on implementing namespaces. I concluded that the modification was too large. I encounterd several (technical debt?) things that were causing additional complexity.
I am now at the point where I want to start over, but first get rid of the things I encontered:

• The prettyprinter/parser roundtrip contains flaws for some edgecases. However, currently they are not detected, because of some shortcuts in the definition of some instances of Arbitrary.
• The datatype for objects (P_BoxBodyElement) are somewhat confusing. This technical debt made me suffer.

I want to fix above things before doing a new attempt with namespaces.