Numbers: axiomatization, properties and implementations of gcd

 - For nat, we create a brand-new gcd function, structural in
   the sense of Coq, even if it's Euclid algorithm. Cool...
 - We re-organize the Zgcd that was in Znumtheory, create out of it
   files Pgcd, Ngcd_def, Zgcd_def. Proofs of correctness are revised
   in order to be much simpler (no omega, no advanced lemmas of
   Znumtheory, etc).
 - Abstract Properties NZGcd / ZGcd / NGcd could still be completed,
   for the moment they contain up to Gauss thm. We could add stuff
   about (relative) primality, relationship between gcd and div,mod,
   or stuff about parity, etc etc.
 - Znumtheory remains as it was, apart for Zgcd and correctness proofs
   gone elsewhere. We could later take advantage of ZGcd in it.
   Someday, we'll have to switch from the current Zdivide inductive,
   to Zdivide' via exists. To be continued...

git-svn-id: svn+ssh://scm.gforge.inria.fr/svn/coq/trunk@13623 85f007b7-540e-0410-9357-904b9bb8a0f7

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