serialization: included duct::IO Cacophony support header.
utility: aliased duct::enum_combine().
IO::PropStateStore: added supplied_conj().
IO::Datastore::create_object(): added check for object existence, removed type restriction,¹ added linkage parameter.

1: The latter was a layover from lack of object type registration.
Cmd::Node::Create: renamed result_data to result; updated & moved create_object() call below construction.