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.
Indeed.  I suspect the best thing to do when we get there might be to pick something to stand for "element of the groupoid", and rename the things in `PathGroupoids` appropriately.  Perhaps even define a type for "a 1-groupoid", and get all of the lemmas as instantiations of the lemmas for that type.