Updated the licence info and header for each file.Moved backends in separate folders
Prepared first stage of code reorg:1. moved type def in lustrespec.ml2. moved constructor and basic functions in corelang3. Modified eexpr with prenext quantifiers
Do not use stable sort because it requires recent ocamlgraphlibrary (1.8.3) which is not widely available in distro repository.Moreover "stable" sort is not necessary, sort will do.Fixes Issue #49: https://cavale.enseeiht.fr/redmine/issues/49
Moved files to trunk in lustre_compiler