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This reverts commit 4c056f5e.

It is just a test case and not really part of the barrier library.

It is not a library; it does not contain code, but instead is a core
part of heap_lang.

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.

Also remove some superfluous map_ prefixes.

They will use Iris is a library, in that paper's repo

The proof is not pretty, though...

Thanks to Amin Timany for the suggestion.

add a version of [cancel] that works with goals of the form [_ |- pvs _]; and use that for the barrier proof

This cleans up some ad-hoc stuff and prepares for a generalization
of saved propositions.