adderall.internal: Use cdr instead of slice

Since (cdr) is available in, and behaves the same in both hy 0.10.0 and
0.10.1 (and master too), use that instead of (rest) and (slice coll 1).

Thanks to Paul Tagliamonte for the suggestion!

Signed-off-by: Gergely Nagy <algernon@madhouse-project.org>