`UnboundLocalError` not implemented

The interpreter currently doesn't check if a variable is being read before being assigned to in a scope.

It leads to bugs like this:

```python
x = 10

def f():
    x = x + 1  # accidentally reading the variable from global, but writing in local
    print(x)

f()
print(x)
```

```console
$ interpreted asd.py
11
10
```

This should throw `UnboundLocalError` instead.