adderall.dsl: Add (prep)

(prep) takes a list of goals, extracts all symbols that start with a
"?", and makes them fresh lvars around the goals. This means that:

(prep (≡ ?x ?y)) => (fresh [?x ?y] (≡ ?x ?y))

Signed-off-by: Gergely Nagy <algernon@balabit.hu>