_isEmpty should respect "emptiness" of the PType

Summary: Currently, if I have Property P of Exactly One Large Text, P -> _isEmpty will always return false -- by definition, Exactly One can't be empty. But that's unintuitive: if the text length is zero, I expect it to return true.

This is a subtle problem, which is why I'm not just doing a quick hack for it. The implication is that there should be what amounts to a typeclass that defines "emptiness", which PTypes can implement or not. Rather than hacking, we should figure out the idiom for defining typeclasses is in Querki, and use that as a kicker for _isEmpty.

We may also want a way to distinguish "strict" _isEmpty -- the current definition, taking only the Collection into account -- and this looser definition, for those occasions when the user gives a damn. Might be a named param.