New CLI syntax for package version

[Ludovic Courtès]

In <http://bugs.gnu.org/19219>, we came to the conclusion that we need a
new syntax to denote a specific package version on the command line.

The current syntax is described in the manual (info "(guix) Invoking
guix package").  Basically, ‘guile-1.8’ refers to version 1.8.x of
Guile; however, this syntax has proved to be ambiguous for packages
whose name contains digits.

For the new syntax, the proposals so far are:

  1. slash, <http://debbugs.gnu.org/cgi/bugreport.cgi?bug=19219#25>

     guile:1.8/doc
     xterm-256-color:320
     emacs:24.5/out

  2. underscore, <http://debbugs.gnu.org/cgi/bugreport.cgi?bug=19219#28>

     emacs_24.5:out

  3. at, <http://debbugs.gnu.org/cgi/bugreport.cgi?bug=19219#31>

     address@hidden
     address@hidden:doc

What do people think?

Ludo’.