rules/collectionᵒ: New ruleset

This adds a new ruleset that does transformations related to
collections. As a start, (get x 0) => (first x).

Signed-off-by: Gergely Nagy <algernon@balabit.hu>
rules/collectionᵒ: Move it a bit further down

Signed-off-by: Gergely Nagy <algernon@balabit.hu>
rules/collectionᵒ: Add a (slice x 1) => (rest x) rule

Signed-off-by: Gergely Nagy <algernon@balabit.hu>
rules/collectionᵒ: Add a (zero? (len coll)) => (empty? coll) rule

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