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.
It tripped me up when I encountered it, too.  @mattam82 might be able to comment on the rationale for the design choice.  I don't recall whether or not the instance is global when inside sections; test it and let me know?  When done at top level, it's automatically exported, so I'd guess that it's global automatically.