Enable and improve GOTO instrumentation

[FrNecas]

In order to make the feature fully work, some minor refactoring was necessary.

I've tested the feature manually on the following examples and all of them seem to work as expected:

• regression/invariants/loop1 (basic test)
• regression/invariants/nested2 (--zones)
• regression/invariants/loop6 (--octagons)
• the following test for heap domain without dynamic objects:

  
  typedef struct data {
  struct data *next;
  int content;
  } Data;

void main() { Data x1 = {0,}; Data x2 = {&x1,}; Data x3 = {&x2,}; Data *curr = &x3; while (curr) { assert(curr->content == 0); curr = curr->next; } }

[FrNecas]

Thanks for the reviews. Braces fixed and basic test coverage added.

[peterschrammel]

I think you can merge @viktormalik

[viktormalik]

Indeed, thanks. Merged.