Experimentation sur l'interface de transform.ml. Par contre quand est ce que le contexte est applati dans theory.ml?

Mise à jour de transform, inlining,flatten,... cependant anomalie car un use peut être ajouté par add_decl...

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the [Duse] declaration are preserved on Context.use_export,
so that ctxt_known can be always restored from ctxt_decls.

- split clone code between contexts and theories

- add [Dclone] declarations after cloned decls

- account for the used theories in [ctxt_known]
  (the hash-consed [Duse] declaration is referenced)

- provide use and clone for contexts with a distinct semantics:

    Context.use_export ctxt th :
        Copies the contents of [th] and its dependencies
        into [ctxt]. The [Duse] declarations are erased
        from the list of decls, but keeped in [ctxt_known],
        so that theories are not copied twice. Useful to
        build flat contexts.

    Context.clone_export ctxt th :
        Clones the contents of [th] into [ctxt]. Dependencies
        of [th] are copied, too. Erases the [Duse] declarations,
        but keeps them in [ctxt_known], so that theories are not
        copied twice. Useful to build flat contexts.

    Theory.use_export uc th :
        Marks the dependency on [th]. [Duse th] is added to [uc],
        unless [th] is already used.

    Theory.clone_export uc th :
        Clones the contents of [th] into [uc]. The dependencies
        of [th] become dependencies of [uc]. [Duse] declarations
        are added, when a theory is used for the first time.

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