Also, I removed the @ from lookup_weaken since the Coq bug we
experienced before somehow disappeared.

Also, add algebra/gset to _CoqProject.

There was not really a need for the lattice type classes, so I removed
these.

These just make things more complicated, it would be more useful to
declare (efficient) decision procedures for each instance, so that
we can properly predict which instance we will get.

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

In particular, remove ectx_language.atomic, because it seems unused
expect for a smart constructor for language.atomic.

Also, remove some unused lemmas about agree.

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