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.
This seems slightly inaccurate, given that we have `Local Instance`; it's not so much "Inside the section" that matters as "make the instance local", whether that means using `Local (Existing) Instance` at top-level, or `(Local) (Existing) Instance` inside a section.