New “guix pull” command

[ludo@gnu.org (Ludovic Courtès)]

Hi,

Commit f651b47 adds a “guix pull” command.  It fetches a Guix source
tarball, unpacks it, compiles Scheme code (so not the daemon), and
changes ~/.config/guix/latest to point to it.

The commit also changes the ‘guix’ command to put ~/.config/guix/latest
at the front of the search path (except when using ./pre-inst-env.)

This way, it allows users to at the same time update package
descriptions (like “apt-get update”) and the package manager itself.

I considered other options, such as making a package for Guix and
letting users install it in their profile.  That would have provided
roll-back and everything for free.  However, I came to the conclusion
that package management of the package manager itself deserves special
treatment, and should not be in the way of users.

The implementation has two main problems: it does not authenticate the
tarball it downloads, and downloads a full tarball (~5 MiB) every time.

We need a bit of infrastructure to fix that.  We could have a process on
hydra.gnu.org that automatically signs tarballs with a pre-defined key.
For the second issue, there are Git, rsync, bsdiff, or even GNUnet that
could help, but that requires more thought.

Comments welcome!

Thanks,
Ludo’.