Greatest common divisor for Nat

[runarorama]

Adds gcd which is greatest common divisor for Nat.

The changes summarized below are available for you to review, using
the following command:

pull-request.load https://github.com/unisonweb/base:.trunk.Nat https://github.com/runarorama/mybase:.gcd

Added definitions:

 gcd                       : Nat -> Nat -> Nat (+3 metadata)
 gcd.doc                   : Doc (+2 metadata)
 gcd.tests.commonDivisor   : [Test.Result] (+3 metadata)
 gcd.tests.multipleOfAnyCD : [Test.Result] (+3 metadata)