List cycles support

Marwood doesn't support list cycle representation and create infinite loop:

```scheme
(define x (let ((x '(1 2 3)))
   (set-cdr! (cddr x) x) x))
> x
```

It also doesn't support R7RS datum reference syntax, which allows for creating cycles:

```scheme
(define x '#0=(1 2 . #0#))
```