nb: still does not work for nested iterators

I think it is  safe to suppose that the node is on  the base clock in
this case.

The dependency on the clock was lost when the enum is translated into
an integer (because some expression were still refering to the enum type).

The fix was to index VarMap.t by string instead of Soc.var_expr in ActionsDeps.

such as in the equation
 o1,o2 = current (n(x,y,z));

In the -2c mode, in presence of extern types, I was defining them as int.
The idea was to have something that compiles, and then to ask the user
to redefine its extern types.

Now I define extern types as string (char*).

The rationale of this change is that before, the generated code was only
working for int, which is pretty useless as an extern type.

Have string by default is actually much more useful, for instance to use
string in a Lustre program.

Also, adapt to changes done in lutils, where the string type has ben added.
(sha:8619aa285ca8322ab9, i.e., > lutils  1.49.10)

(It would have avoided issue #8)