Return whether the given placement is a valid placement of small_graph onto big_graph.

This is done by making sure all the nodes and edges on the mapped version of small_graph are present in big_graph.

big_graph A larger graph we're placing small_graph onto.
small_graph A smaller, (potential) sub-graph to validate the given mapping.
small_to_big_mapping A mapping from small_graph nodes to big_graph nodes. After the mapping occurs, we check whether all of the mapped nodes and edges exist on big_graph.