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).

Both ndot and nclose involve encodings of countable types, and conversion
should thus never unfold these definitions.

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.

The intropattern {H} also meant clear (both in ssreflect, and the logic
part of the introduction pattern).

Lifting lemmas : make the initial state appear under an existential under the view shift. This is still sound, but slightly more expressive.

In particular, it no longer uses set_solver (which made it often slow
or diverge) but a more specific lemma about subseteq.

Concretely, when execution of any of the wp_ tactics does not yield
another wp, it will make sure that a view shift is kept. This
behavior was already partially there, but now it is hopefully more
consistent.

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

Fixes issue #20.

This introduces n hypotheses and destructs the nth one.

We used => before, which is strange, because it has another meaning
in ssreflect.

And use slice_name, which is defined as gname but Opaque, instead
of gname in boxes.

They mess up the proof mode notations due to overlaps.