Instead of having connectives pvs0 and pvs1 we now have one connective
pvs that is indexed by a Boolean.

Also make those for introduction and elimination more symmetric:

  !%   pure introduction         %        pure elimination
  !#   always introduction       #        always elimination
  !>   later introduction        > pat    timeless later elimination
  !==> view shift introduction   ==> pat  view shift elimination

1.) iDestruct is able turns  into two implications (because uPred_iff
    is (type classes) transparent).
2.) iApply only backtracks on turning P  Q into P → Q or Q → P when
    there are no future premises. This is not the case for
    'P  □ (P → False)'.

It not behaves more consistently with iExact and thus also works in the
case H : P -★ □^n Q |- Q.

show that even \later^n False is inconsistent (for any fixed n); properly use pvs in counter_examples