Hard fork history tests.

This is the more interesting test of the hard fork history. We construct a mock chain, consisting of events (events are roughly, but not quite, "blocks"). For every event we record its slot number, epoch number, wall clock, etc. Since we are constructing this chain as a whole, from genesis to its tip, constructing these events is trivial. We then split this chain in half, and construct a Summary from the first half. We then use that summary to do conversions for any event on the chain. Since every event records all information, we can easily verify whether the answers we are getting back are correct. Moreover, since the summary is constructed from only the first part of the chain, but is used to do conversions across the entire chain, we verify that predictions about the "future" also work as correctly (including that the conversions say "outside range" if and only if the model expects them to be).