Potential nextgen layering support

[HyperCodec]

I made a plotters nextgen in my neat crate example that would run another nextgen once it was done collecting the diagnostic data. It would be very nice if this could become a builtin feature or trait (maybe even with tuple layering support).

[HyperCodec]

I could probably do this by simply blanket implementing NextgenFn on tuples of nextgens and then piping the generations.

[HyperCodec]

With #96 finally merged, I believe this was implemented in one way. However, the plotters functionality remains unimplemented. Since all that's left is really observers, I think I should make another issue with more specific implementation details.