Add some tests with various kinds of expr that we can have after a when.

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

are defined exactly once.

This  algo is  naive and does not  work with  slices of  slices with
negative steps.  I commit it  before trying a  new one to  get better
non-reg test.

Fix some non-reg test in the should_work dir that this new check revealed.

It  does not  work, I  have changed  my mind  to use  a  more general
clocking algorithm based on  unification. I commit that current state
just in case I change my mind again...