Moved backends in separate folders

   - typing/clock calculus/normalization now work properly
   - still, a bug in annot generation (this one is for Ploc !!)
     in file normalization, line 396
 - bug corrected in subtyping
 - scheduling outcome changed:
   - variables appearing in a call output are gathered
   - ease the design of a reusability algorithm
 - liveness/reusability is still a work in progress

Merged branches specification_reorg_corelang_parser (see last commit message, moved definitions/functions btw files and changed eexpr type)

1. moved type def in lustrespec.ml
2. moved constructor and basic functions in corelang
3. Modified eexpr with prenext quantifiers

 could be useful for a finer variable elimination scheme at the Lustre level.
 to be continued...

  - exact definition of 'useless' may be further refined
  - display could certainly be improved

 - warnings for unused input/memory variables
 - optimization of machine code

Still some work on adapating the instruction scheduling

Merge Xavier last commits
Unfinished lustre backend

   in C code generated from relational operators.

 - simplification of clock calculus (may be still buggy, work in progress)
   no impact on unclocked programs.