concat always returns a vector

Pixie's concat always returns a vector, whereas in Clojure, it returns a lazy seq:

Pixie:

``` clojure
(concat (take 2 (range 1)) (take 1 (repeat nil)))
;;=> [0 nil]
```

Clojure:

``` clojure
(concat (take 2 (range 1)) (take 1 (repeat nil)))
;;=> (0 nil)
(realized? (concat (take 2 (range 1)) (take 1 (repeat nil))))
;;=> false
```

I think the Clojure behaviour is more desirable here.
