Summary:
Merge utils/visitor directory into single file.
Rename Visitor to HierarchicalVisitor.
Add regular Visitor.
Split HierarchicalVisitor into LeafVisitor and CompositeVisitor.
Add more documentation on visitor pattern.
Make PostVisit and Visit return bool.
Reviewers: florijan, mislav.bradac, buda
Reviewed By: mislav.bradac
Subscribers: pullbot
Differential Revision: https://phabricator.memgraph.io/D364
