Re-visit perf of letfn

[borkdude]

See #395

We could do the equivalent of something like this:

(defmacro letfn_ [fnspecs & body]
  (let [syms (map first fnspecs)]
    `(with-local-vars ~(vec (interleave syms (repeat nil)))
       ~@(map (fn [sym fn-spec]
                `(var-set ~sym (fn ~sym ~@(rest fn-spec)))) syms fnspecs)
       nil
       ~@body)))

Then we would need to implement with-local-vars first. See #383. If that doesn't perform the way it should, then we could re-implement it as a special form.

[borkdude]

Fixed:

$ time ./sci "(letfn [(a1 [n] (if (#{0 1} n) 1 (+ (a2 (- n 2)) (a3 (- n 1)))))
        (a2 [n] (a1 n))
        (a3 [n] (a1 n))]
  (a1 30))"
1346269
./sci    4.16s  user 0.10s system 99% cpu 4.266 total

$ time /usr/local/bin/joker -e "(letfn [(a1 [n] (if (#{0 1} n) 1 (+ (a2 (- n 2)) (a3 (- n 1)))))
        (a2 [n] (a1 n))
        (a3 [n] (a1 n))]
  (a1 30))"
1346269
/usr/local/bin/joker -e    7.41s  user 0.74s system 148% cpu 5.478 total