adderall.dsl: Fixed a bug in consᵒ

The default case for consᵒ was erroneously trying to unify (cons f [r])
with l, instead of unifying it with (cons f r).

Reported-by: Nicolas Dandrimont <nicolas.dandrimont@crans.org>
Signed-off-by: Gergely Nagy <algernon@madhouse-project.org>
tests: Enable more tests from The Reasoned Schemer, chapter 3

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