import: crate: Allow imports of a specific version.

* guix/import/crate.scm (crate->guix-package): Add optional 'version'
argument and honor it.
* guix/scripts/import/crate.scm (guix-import-crate): Assume the first
argument is a spec and destructure it with
'package-name->name+version'.  Pass both to 'crate->guix-package'.
* doc/guix.texi (Invoking guix import): Document it.

Co-authored-by: Ludovic Courtès <ludo@gnu.org>