Add class for SystemDependenceGraph

[mchalupa]

We still can build just dependence graphs without any hierarchy, but this class will make easier building of the whole-program graphs

[mchalupa]

Fix together with #248 .

[mchalupa]

So we've got the representation, but there are still some features missing:

• [x] SDG representation
• [ ] building SDG for LLVM
  • [x] intraprocedural
  • [x] interprocedural with interprocedural edges
  • [ ] interprocedural with parameter edges
• [ ] slicing SDG
  • [ ] one-phase slicing
  • [ ] two-phase slicing
    • [ ] summary edges
    • [ ] slicing itself
  • [ ] slice LLVM according the the sliced SDG
• [ ] generate multiple slices of the same SDG