Hash function for individual traversal states (node visits)

Whenever we are at a node during traversal, we need to have a unique hash value for that particular traversal state. In other words, if we are at this node (not neccessarily SIDs here):

alpha/bravo/charlie

we want a unique hash that is different from when we are at this sibling node:

alpha/bravo/delta

Then, when we are later back at

alpha/bravo/charlie

on a subsequent traversal, we have the same hash value again, from which we can determine that we are back at that particular location.

This is to help the issue with correct event handling within Instance-Symbol relationships: #142 Pick event routing broken across Instance-Symbol links