Bug fixed in handling of explicit named substitutions: it could happen that

Bug fixed in handling of explicit named substitutions: it could happen that
explicit named substitution are merged not in the expected order. This happens
comparing the case of an instantiation inside and outside a (nested) section.
Thus I have added back the re-ordering of the explicit named substitution in
CicSubstitution.subst_vars and in CicReduction.unwind_aux. Moreover, I have
added to CicTypechecker.type_of_aux' the check that verifies if an explicit
named substitution is ordered.