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.
Ah, that explains why it wasn't coercing when I expected it to!  That seems like a poor design choice to me, to have `:>` mean one thing in a record and a different thing in a class.  Is the resulting instance automatically global?