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.
Unsurprisingly, I didn't discover this behavior by reading the refman, but by trying to figure out why my code wasn't working.  (-:  It could generously be interpreted as a consequence of the sentence "Any argument not given as part of a type class binder will be automatically generalized", but it does seem to contradict the claim that `Generalizable No Variables` "disable[s] implicit generalization entirely", since clearly some implicit generalization is still happening.  Maybe the ambiguity is in the meaning of "free variable"?  In any case, whether or not the current behavior is the intended one, the refman ought to say explicitly what happens in this situation.