enum nor a bool.

to avoid priority errors.

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

compact, cleaner, and correct wrt negative steps in array slices.

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.