Boxes are now grounded implicitly by Setup::Entails().

We also could instantiate boxes with sequences of action variables, but
that doesn't look more efficient because it adds a lot of new variables
(which leads to bigger H+) and leads to many unifications, which
perhaps are not necessary because usually we have only one or two
different action sequences.
H+ is kept in sync implicitly by Setup::AddClause().

That is more expensive than requiring the user to call
Setup::UpdateHPlus(Term::Factory) once, so we might return to the old
pattern. (Then, be careful not to do c.CollectNames() in
Setup::AddClause().)