We should probably start importing these in `nengo.__init__` because they're now pretty much essential (and there's very little cost importing these, they're just class definitions). There's two questions related to that:

1. Do we import both `nengo.objects` and `nengo.nonlinearities` (I would say yes).
2. Do we import these namespaces, or the contents of them (i.e., do we want people to write `nengo.objects.Ensemble` or `nengo.Ensemble`). I would say the latter.