PointsToGraph: pass ProgramStructure pointer

instead of reference. Refrence invokes copy constructor.
Now it's not used, but later I'd like to use it for building
the whole graph from constructor
PointsToGraph: add insertDerefPointee

This function takes the same arguments as insert, but do dereference
of location. That is, instead of adding 'p points to location' add
that 'p points to everywhere where can location point to'.

I also had to add few helper methods (well, didn't have to, but the
code is more lucid)
PointsToGraph: add insertDerefPointer

the same as insertDerefPointee, but now is the pointer dereferenced, not
the pointee
PointsToGraph: add forEachFromNode

run particular function on each pointee in node
PointsToGraph: use const ProgramStructure

We don't change program structure..