maintaining (ugly and error-prone) hash tables.

That change revealed an untriggered bug in EvalClock.check_args: it was wrong
to add in subst the substitutions made of the parameters and the arguments (it
is enough to unify the clocks of the pars and of the args). For instance, consider
the node (in should_work/clock/clock.lus)

   node clock5(x :  bool; y:  bool when x; z: bool when y)

and the call

  z2 = clock5(a, b when a, c when e);

I was adding y/b in the subst, which was wrong.

Other minor changes:
 - move const_to_val_eff from Eff to UnifyClock.
 - GetEff.translate_val_exp now returns a substitution, in order to be able
   to unify clock vars and propagate the resulting substitution.

constants that  are not  on the base  clock, add the  suitable "when"
statement if it was not present  in the source code (the lic (and the
ec/lv4) backend  does not  perform such kind  of clock  inference for
constants.

change the parser and accept only clock expressions after a when.

A clock is now made of 2 idents: one for the clock constructor, and
one for the clock variable.

PredefEvelClock, in a similar manner as for type checking.

clock_info, and  therefore also  remove clock_info. The  rationale of
this  change is  that  it  makes things  slightly  simpler, and  more
homogeneous with what is done in type checking.

Not yet implemented (assert false): iterators, struct

Add a UnifyClock module, and rename Unify into UnifyType.

nb : a lot of test are now broken, because
   - the clock checking is now plugged ;-)
   - iterators, struct are not yet implemented