Fix let to make recursive functions possible

![](http://i.imgur.com/BMgNSsj.gif)![](http://i.imgur.com/BMgNSsj.gif)<br>![](http://i.imgur.com/BMgNSsj.gif)![](http://i.imgur.com/BMgNSsj.gif)
So, now the environment is always passed by reference. This poses a problem...