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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 is more consistent with the proofmode, where we also call it pure.

Par is now defined as an expression of type [∀ X, expr X] (instead of a
value) and we prove that it is stable under weakening and substitution.

That way, we do not have useless type annotations of the form
"v : language.val heap_lang" cluttering about any goal.

Note, that we could decide to eta expand everywhere (as we do for ∀
and ∃), and use the notation "WP e {{ Q }}" for "wp e ⊤ (λ _, Q)".