We're probably going to want to change some of these design decisions down the
road, but this is a good starting point.
* We may want to use a more general test for "is actor valid at epoch". Maybe
just a function?
* I'd like to push some of the actor metadata down into the actor types
themselves. Ideally, we'd be able to register actors with a simple
`Register(validation, manyActors...)` call.
