of "red<<if, 3>>".

Put the unification stuff in  a dedicated module, and add some random
unit tests to it.

Add Some lustre files that are automatically tested thanks to the
generic Makefile (from the Youssef compiler).

Test files are organized as follows :

 - files in directories that are under the "should_fail" directory
  do triggers errors, but it is intented

	should_fail/syntax
	should_fail/type
	should_fail/semantics

 - and files that should not trigger any error:

	should_work/