* Migrated `NodeType`s to specific `GraphNode` child classes.
* Revamped creation of SDG
* Added ASDG and PSDG, based on the APDG and PPDG.
* Improved handling of variable usages, definitions and declarations.
* Added movable variable actions, that can be placed in a CFG node and
  then relocated to their own PDG node.
* Added fix-point and specific slicing algorithm to summary arc
  generation.
* Documentation of most methods (public and private).
* Removed unused elements.
* Fix possible errors by migrating all maps whose key is an AST node
  to IdentityHashMap.