(which are useful in Amin's logical relations development).

This generalization is surprisingly easy in Iris 3.0, so I could not
resist not doing it :).

* Values are considered as atomic expressions (this does not hurt, and
  this makes the proofs of atomicity simpler).

I also reverted 7952bca4 since there is no need for atomic to be a
boolean predicate anymore. Moreover, I introduced a hint database
fsaV for solving side-conditions related to FSAs, in particular,
side-conditions related to expressions being atomic.

and show that this is an instance of evaluation contexts