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.
Yeah, the evar meaning of `eapply`, `erewrite`, `eassumption` is pervasive.  We could call it `pmove` for pointwise move, since the only thing that is special about these lemmas over the general groupoid ones is that you only ask for the hypotheses to be true pointwise, rather than of the entire functions.  That might be confused with `path_move`, though.  I'm not sure I care too much, though, and happy with `emove`.  Another possibility is `equiv_move`.  I suppose `movee` is also a possibility.