Debugger improvements

[marcoeilers]

• Heap labels include line numbers again; the new format we show is old[debug@4#l:13.11]([email protected]) instead of just old[debug@4]([email protected])
• Internally, the heaps are only identified by the debug@i part, and it is sufficient to use that part in the debugger when entering expressions, but one can also use the full label name.
• Non-QP field chunks now contain a value expression, which is also shown in the debugger
• Added the option to print the internal term representation of the store, heap, assumptions and branch conditions instead of the Viper-level expression representation