Add a `Fast` wrapper class

Provided for convenience, allows `fast` methods to be chained.

```js
var arr = fast([1,2,3,4,5,6]);
var result = arr.filter(function (item) {
  return item % 2 === 0;
});
result instanceof fast; // true
result.length; // 3
```
Just extend Array, it'll be a terribly evil fun