Make Predictions have an independent existence

[elidupree]

Currently, Event implementors are required to maintain the invariant that the predictions known by the TimeSteward are the same as the predictions accessible from Globals. This is a tricky condition that could easily be broken by mistake. Currently, you can rely on SimpleTimeline to handle it accurately, but that also has performance overhead, and the SimpleTimeline code has a big cost in maintainability.

There was once a reason for this behavior – making predictions implicit in the state of Entities meant that the representation of the canonical state was less complicated. I kept this when I got rid of Predictors, but it no longer seems worthwhile. Events should explicitly create and destroy Predictions.

Code-wise, the changes might not be that big. This does mean that I will have to change the serialization code a bit. I can also remove the reference-counting from predictions. And I might need to think about what happens when a predicted event actually occurs (e.g. is the prediction implicitly destroyed?).