Fix non order-only dependency of output binaries on output directory.
Fixes unnecessary linking when only one binary needs to be re-linked.
Make Handler (JSON serialiser) virtual, to allow optional pretty-printing.
Rename class world_serialisation to world_deserialisation.
Store whether a world_object name is automatically assigned.
Make test_fixture_world movable/move assignable.
Now uses unique_ptrs to store world_test, world_deserialisation,
instead of in-place storage.
Add serialiser flags to control world_obj name serialisation.
Add a world_serialisation class with a SaveGameToString method.
De/serialise game time and load count.
Fix future deserialisation.
Add mechanism to clone test_fixture_world by serialisation round-trip.
Allow test_fixture_world to be initialised with separate content and
gamestate strings.
Port points movement/futures test to also test deserialisation round-trips.