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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.

We reify to a representation of expressions that includes an explicit
constructor for closed terms. Substitution can then be implemented as
the identify, which enables us to perform it using computation.

This gets rid of the (ambiguous) notation %l, because we can declare
LitLoc as a coercion. It also shortens the code.

and show that this is an instance of evaluation contexts

This required a new ectx axiom: Positivity of evaluation contexts. This axiom was
also present in the old Iris 1.1 development, back when it still derived lifting
axioms for ectx languages.

Thanks to Amin Timany for the suggestion.

For consistency's sake.

get rid of substitution in Case (use lambdas); introduce Match as derived form that involves binders

We no longer abuse empty strings for anonymous binders. Instead, we
now have a data type for binders: a binder is either named or
anonymous.