Implement Flatten system function

```
See more here: http://www.aplusdev.org/APlusRefV2_14.html#HEADING286

Exmaple:

_flat{(iota 3; < 2 3; 1 2 rho 6)}

Result:

0 1 2 2 3 6 6
```

Original issue reported on code.google.com by `batori.c...@gmail.com` on 6 Jun 2011 at 1:29
