Enhance map views in debugger interface

[NatKarmios]

Resolves #176 Resolves #170

• Allow nested exec/unify maps to be collapsed
• Render unification folds as nested maps
• Absorb Success/Failure unify nodes into the previous assertion

[NatKarmios]

We really need a way of testing this, or simplifying the review of this kind of code

This is #177. Can make it a priority if need be :)

[giltho]

Priorities will be assessed when GJS is fixed