import: Add CHICKEN egg importer.

* guix/import/egg.scm: New file.
* guix/scripts/import/egg.scm: New file.
* tests/egg.scm: New file.
* Makefile.am (MODULES, SCM_TESTS): Register them.
* po/guix/POTFILES.in: Likewise.
* guix/scripts/import.scm (importers): Add egg importer.
* doc/guix.texi (Invoking guix import, Invoking guix refresh): Document it.

Signed-off-by: Ludovic Courtès <ludo@gnu.org>

po/guix/POTFILES.in

@@ -7,6 +7,7 @@ gnu/system.scm
 gnu/services/shepherd.scm
 gnu/system/mapped-devices.scm
 gnu/system/shadow.scm
+guix/import/egg.scm
 guix/import/opam.scm
 gnu/installer.scm
 gnu/installer/connman.scm
@@ -100,6 +101,7 @@ guix/scripts/environment.scm
 guix/scripts/time-machine.scm
 guix/scripts/import/cpan.scm
 guix/scripts/import/crate.scm
+guix/scripts/import/egg.scm
 guix/scripts/import/gem.scm
 guix/scripts/import/gnu.scm
 guix/scripts/import/go.scm