- added interface files (.lusi) in the language, that can be compiled on their own, giving an object file (.lusic) and a header file (.h)
  - modular code generation, from Lustre to C level included.
  - nice amount of code refactoring

   in order to ease the way .lusi interface files are handled.

   much more simple and efficient.

   weak (but still correct) optimization results.

   tuple expressions
 - added an algorithm that reuses dead or clock-disjoint 
   variables instead of declaring/using new ones.
   - NOT carefully tested. Use option -O 3 if you want 
     to give it a try
   - some output name may change from node to machine code,
     this point could be discussed...
   - further optimizations of machine code are now enabled
     (eg. merging case constructs)

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