Binary substituter

[Ludovic Courtès]

Hello,

Commit f65cf81 adds the beginning of a “binary substituter”.

The substituter (in Nix parlance) is the mechanism by which a pre-built
binary can be substituted to a locally-built binary.  The idea is that,
when you’re about to build, say, /nix/store/xxx-emacs-24.3.1, the daemon
ask substituters whether they have substitutes for that; in turn,
substituters typically ask some server whether it has that path and
download it.  Thus, it allows transparent binary deployment, building
from source when no other option is available (or the user insists).

This commit adds the ‘guix substitute-binary’ command, which is intended
for use by the daemon, not by normal users.  So far, the daemon can
invoke it to know which substitutes are available on hydra.gnu.org.  It
cannot actually download them yet, but that should be done Real Soon, so
stay tuned.  :-)

Ludo’.