Consider adding a generic `Alias` type

This would allow a datastream to reference an already existing data stream elsewhere in the dataset. Some questions:

- Should likely be a `Alias[TDataStream]`
- Should not change the way data streams load lazily
- How should it load the target datastream if not loaded yet? Should it "force" initialization of intermediary streams that may be necessary?
- How should it be initiated/referenced?
- Should it be validated at construction time at all?
- Should not be able to reference itself