Fix JSON serialization of visited_instances
Fix JSON serialization of visited_instances

OrderedDicts get JSON serialized as regular dictionaries, so the
de/serialization process was causing a type error.

Instead, we just convert to and from OrderedDict and store as a simple
but readily compatible type, lists of tuple pairs.

RickMohr/otm-core,maurizi/otm-core,clever-crow-consulting/otm-core,recklessromeo/otm-core,recklessromeo/otm-core,clever-crow-consulting/otm-core,recklessromeo/otm-core,clever-crow-consulting/otm-core,maurizi/otm-core,maurizi/otm-core,recklessromeo/otm-core,maurizi/otm-core,RickMohr/otm-core,clever-crow-consulting/otm-core,RickMohr/otm-core,RickMohr/otm-core