Summary:
Replace includes in operator.hpp with forward declarations.
Provide basic documentation for remaining operators.
Use SwitchOld on accessors used in Expand and Filter operations.
Add SwitchOld and SwitchNew to ExpressionEvaluator.
Evaluate new or old state in operators.
Test operators use correct accessors.
Add some basic tests for cases where switching accessors to old and new
matters.
Reviewers: mislav.bradac, buda, florijan
Reviewed By: florijan
Subscribers: pullbot
Differential Revision: https://phabricator.memgraph.io/D208