ast modifs and additional arrow_taint phase to retrieve which arrow "protects" / "taints" which variable

node_Table hashtbl is now only available through functions of the corelang.mli

- Renamed type declarations as lustre_types and machine_code_types

[general] large modification: added machine types, a second typing phase dealing with machine types (eg uint8)
          typing was transformed as a functor and parametrized by basic types (int/real/bool)
          it can also be applied multiple times on the same program