Hello all,

Here's a preliminary patch that does two things:

* Changes 'guix-build' and 'guix-package --install' so that only the
  newest packages will be considered (unless a version number is
  specified).

* Implements 'guix-package --upgrade'.

Although I'm not aware of any functional problems with this code, I'm
not entirely pleased with its organization.  Nonetheless, I wanted to
make it available for early testing and comments.

I welcome suggestions on how to improve this code.

      Mark