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'm curious to hear the rationale behind this naming convention.  `_VE` doesn't seem to extend well to whatever the analogue of `_Vp` and `_pV` and `_pM` and `_Mp` would be, though maybe those are less necessary with equivalences.