Debugging improvements

[marcoeilers]

• Two changes to make sure we keep old (debug) heaps recorded in the state during expression evaluation, which were previously discarded because expression evaluation was not expected to generate additional old heaps
• Changing debug expressions for function applications, unfoldings, and applying expressions from a generic joined_unfolding/... variable to the actual evaluated expression wrapped into an old

[marcoeilers]

@jackstodart This doesn't preserve the old heaps from before a retry yet, but it should fix the rest.