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.
Because people like me will spend an extra 10 seconds staring at the code trying to figure out why `generalize dependent` is needed?  :-) It's not particularly important.