Handling Circular Structures

This is going to come up quickly, but we need to think about how to handle circular structures.

``` js
var minim = require('.minim');

var arr1 = new minim.ArrayElement();
var arr2 = new minim.ArrayElement();

arr1.push(arr2);
arr2.push(arr1);

console.log(arr1.toRefract());
```

This results in `RangeError: Maximum call stack size exceeded`.
