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
```
most of all I don't really like the idea of people leaking these `fast` objects out into the rest of the world. If someone treats a `fast.Array` as a normal `Array` then that will encourage people to return these instance from library methods etc. This *will* end up breaking code further down the line and probably encourages irresponsible usage of the library.