Register : Essaie d'utilisation de WeakHashtbl mais comme 'a registered est polymorphe il ne peut instancier t

the underlying term/fmla is not changed

In the majority of cases, it is just None. 

Update the whole shebang to use the new core modules

- env-dependent functions are not needed any more

- the member function [clear] is not needed, it is
  the responsibility of Driver to drop and recreate
  transformations whenever desirable. 
  
  (François, don't get mad, please :)

Not only Theory, but also Task and Transformation 
do not need to depend on environment. Moreover,
we don't have to track the clone history in tasks.

The only thing we need to do, is to provide three
registration functions in Driver, namely:

  val register_transform : string -> (unit -> task_t) -> unit
  val register_transform_env : string -> (env -> task_t) -> unit
  val register_transform_clone : string -> (env -> clone -> task_t) -> unit

and then another three for task_list_t.

Then any particular transformation that is going to depend
on environment or clone_history, must register itself via
the appropriate registration function. It will be the
responsibility of Driver to recreate the transformation
for every new environment and/or clone history. It will
be easy, given that both [env] and [clone] are physically
comparable and provide a unique tag.

Thus, the generic interface provided by Transform can be
completely independent on [env] and [clone].

This commit implements the proposed interface of Task
and moves the environment stuff into a separate module.

ajout de la transformation split_to_cnf qui met en une forme presque cnf. Une CNF où on garde la position des négations et implications. L'interet est au moins pour la visualisation. On retrouve (normalement) une cnf simplement en poussant les négations.

main.ml : possibilité de mettre specifier plusieurs theories ou buts
split_goals : ajout d'un argument supplémentaire pour ne selectionner que certains buts.

Theory : Correction de l'exception renvoyé quand une théorie n'est pas trouvé par la fonction retrieve.

The proposed architecture is as follows:

- Decl provides the type of declaration, decl, which is a sum
  of type_decl, logic_decl, ind_decl, and prop_decl. decl is 
  h-consed and does not depent on theory, context or anything else.

- Theory provides the types of theories and theories under construction.
  A theory is a list of (decl | use | clone) with namespace, and the set
  of known idents. Theories are not h-consed and they do not depend on
  env or task.

- Task provides the types of environment (env), clone maps (clone), and
  the task itself. It might be interesting to merge Transform with Task.
  An environment stays as it is today. A clone map is a private record 
  with the unique tag and physical equality. A task is essentially what
  context is today:
    
    task_decl  : decl               (* top declaration *)
    task_prev  : task option        (* the previous context *)
    task_known : (ident, decl) map  (* the set of known symbols *)
    task_clone : clone              (* the clone map *)
    task_env   : env                (* the environment *)
    task_tag   : int                (* the unique tag *)

  Tasks are h-consed. There is a guarantee that task_env and task_clone
  of task and task.task_prev are physically equal.

  Unless there is a good reason to do otherwise, the only way to produce
  a task is by split_goal, which takes a theory (and optionally a number
  of goals in it) and creates a list of tasks. Note that sharing IS LOST
  whenever two goals are separated by a clone instruction. However, the
  declarations will still be shared.

- Trasformation works on tasks, producing task lists, tasks, and alphas.

- use_export and clone_export may be allowed on tasks, rebuilding the
  whole task, whenever necessary.

This commit just adds the Decl module, but does not make anything use it.

dans les drivers il n'y a qu'une seul liste de transformations

- assure generation of new variables on create_ls_defn