Clean up the existing MRS package. This means to move non-integral functions from classes to modules (e.g. xmrs.find_subgraphs_by_preds() instead of Xmrs.find_subgraphs_by_preds()), make the API functions consistent (e.g. whether to return a list or a generator, when to return nodeids vs component structures, etc.), make serializers use the same method signatures (maybe abstract the serializer API out of the serializers themselves, and instead put into the mrs package), etc.