Use faster assoc in let

[borkdude]

Seems worth it:

$ time ./sci "(loop [x 1] (let [_x 1 _y 1 _z 1 _y 1] (if (< x 1000000) (recur (inc x)) [_x _y])))"
[1 1]
./sci    1.30s  user 0.08s system 99% cpu 1.389 total

$ time tmp/inline-fns "(loop [x 1] (let [_x 1 _y 1 _z 1 _y 1] (if (< x 1000000) (recur (inc x)) [_x _y])))"
[1 1]
tmp/inline-fns    1.53s  user 0.08s system 99% cpu 1.621 total