Implement setting environment variables

Currently using `System.Environment.setEnv` crashes with this:

```
uncaught exception in Haskell main thread: ReferenceError: h$putenv is not defined
ReferenceError: h$putenv is not defined
    at h$$mi (/Users/mtolly/git/setenv/dist/build/setenv/setenv.jsexe/all.js:38868:11)
    at h$mainLoop (/Users/mtolly/git/setenv/dist/build/setenv/setenv.jsexe/all.js:10661:25)
    at /Users/mtolly/git/setenv/dist/build/setenv/setenv.jsexe/all.js:11127:13
    at /Users/mtolly/git/setenv/dist/build/setenv/setenv.jsexe/all.js:13454:17
    at Object.wrapper [as oncomplete] (fs.js:521:5)
```

So I implemented that apparently missing function :)