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.
I guess one slight drawback to `emove` is that someone might mistakenly guess that the prefix `e` has something to do with its meaning in tactics like `eapply`.  I'm not sure whether I care about that.