rules/optimᵒ: New (defn foo (x) ...) => (defn foo [x] ...) rule

While (defn foo (x) ...) works by accident, it is neither idiomatic, nor
correct. Switch it to (defn foo [x] ...) instead.

Requested-by: Nicolas Dandrimont <nicolas.dandrimont@crans.org>
Signed-off-by: Gergely Nagy <algernon@madhouse-project.org>