ptg: use hastable instead of set

We'll use that space anyway and it's faster. Moreover, it will
allow to get points-to set of a pointer easily from the pointer
ptg: introduce getNode function

Until now, finding out if a node exists and creating new in the case
it doesn't required accessing Nodes 2 times. This function returns
existing node and in the case it had not existed before it creates
one in one access.