Also, add algebra/gset to _CoqProject.

Similar files (gmap, listset, ...) were already in singular form and
matched the name of the set/map data type.

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...