git commit.

   Sligtly generalise  x_check, and  x_check_interface to be  able to
   define      tabulated      version      of     node_check      and
   node_check_interface.  More   precisely,  add  2   fonctions  that
   extracts the name and the package name of a x_key.

   Ditto for lookup_x_eff.

src/lazyCompiler.mli:
   s/do_node/node_check/

Ident.t is now an abstract data type (instead of a string).
fullid is turned into an Ident.long, and is also made abstract.

Add basic regression tests.

Add "assert false" statements everywhere it was necessary.

Add the fby operator in the syntax.