- the symbols that appear in the metas are identified in the xml by
    their position in the task:
    - in which declaration
    - in which definition (if that apply otherwise -1)
    - in which constructor(or case in inductive predicate) (if that apply otherwise -1)
    - in which field (if that apply otherwise -1)

  - the md5sum of the prefix of the task that end with the declaration is used to know if the
    symbol have been changed, and if it is obsolete.

  - currently metas that contains obsolete symbol are removed.

   - add functions to Hashtbl.S
   - without ocaml 3.12 its not possible to do that for Hashtbl without
     copying the signature
   - "open Stdlib" hides the polymorphic hashtbl, perhaps a good idea
     since that avoid errors.

note: program extraction IS NOT yet implemented

Provide nine transformations: split_(goal|all|premise)_(full|right|wp).
split_*_full splits as far as it can, split_*_right produces linear
number of subformulas, split_*_wp stops at the "stop_split" label.
The name "split_goal" is kept for compatibility with older session
files and denotes the same transformation as "split_goal_wp".

Thanks to Johannes Kanig for the suggestion.

The full name includes the library path to the theory/module (if any),
and the qualifier prefix of the ident in the theory/module.

For realization, code extraction, metas-in-sessions, etc, we need
absolute names for identifiers. To that purpose it is better to
have and store the qualified path to every ident.

Provers will have native polymorphism support long before people will
start writing third-party polymorphism encoding procedures for Why3.
(And it's not that I have high hopes about the former.)

and Debug.register_stop_flag into register_flag. While information
flags (selectable by --debug-all) are more common, it is safer to
treat a generic debug flag as behaviour-changing by default.

comportement was not very clear if an exception was raised.

TODO:

  - make the "on hover" prettyer (Find the size used by gtk for the
    popup message en set pp_set_margin to this size?)

  - Some transformations (ex: smoke_detector_*) should we mark them
    specialy during the registration i norder to warn the user?

 (metas, debug flags, transformations, formats) except for label.

This description is used in --list-*. The description can use any of
the formatting markup of Format "@ " "@[",...

Transformations can also specify from which metas and labels they
depend, and add informations about how they are interpreted.

TODO:
  - complete and correct the documentation
  - when a transformation use Trans.on_meta, it should be possible to
    add an interpretation of the metas in the documentation.
  - recover a summary version of --list-* ?
  - be able to export in latex?

  session pairing doesn't compute anymore the shape of the goal, it is
 done before. It was able to compute the shape only when the checksum
 of the task was different, but computing the checksum of the task is
 way more time consuming than computing the shape of the goal (and
 include it).

 So this commit simplify greatly the function and theoretically
 augment just a little the time spent. Experimentaly it's the inverse
 on max_matrix. Until "update_session: done" with or without modifying
 the checksums:

            before     |   after
without : 0.21-0.22 s  | 0.16-0.17 s
with    : 0.23-0.26 s  | 0.18-0.20 s