(in case of a prover upgrade)

  - 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    symbol have been changed, and if it is obsolete.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.