Assignment

We should use [beniget](https://github.com/serge-sans-paille/beniget) for assignments and forward substitution. By using beniget we could allow this kind of functions:

```python
def foo(value):
    if value < 3:
        b = 3
    else:
        b = 4
    return b
```
This should turn into an equivalent of `value < 3 ? 3 : 4`