list is immutable?

e.g.
```
$ ./chickenclisp 
=> (set x '(1 2 3))
[1, 2, 3]
=> (array-set-n x 0 4)
[4, 2, 3]
=> x
[1, 2, 3]
```
`x` is unchanged.

while in  Lisp (SBCL), list is mutable:

```
* (defvar x '(1 2 3))
X
* x
(1 2 3)
* (setf (car x) 4)
4
* x
(4 2 3)
```
`x` is mutable.
