Uniformize reflective subuniverses and modalities

And also add a few more examples.  This closes #457.  The use of typeclasses is a bit hackish (see comments in `ReflectiveSubuniverse.v`), but it's the best way I've been able to think of to avoid having to pass the modality as an argument to everything all the time when there is only one modality hanging around.
This comment clashes with my understanding of how generalizing binders work.  I thought
```coq
Definition bar `{foo} ...
```
was equivalent to
```coq
Definition bar {H : foo} ...
```
but your comment seems to suggest this is not the case.