Motivation:
We wish to separate these two into clearer write/read interfaces.
In particular, we don't want to be able to add listeners to promises, because it makes it easy to add them out of order.
We can't prevent it entirely, because any promise can be freely converted to a future where listeners can be added.
We can, however, discourage this in the API.
Modification:
The Promise interface no longer extends the Future interface.
Numerous changes to make the project compile and its tests run.
Result:
Clearer separation of concerns in the code.
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