Evaluating a long list with eval() causes segfault.

See the following example, we take a long computation of list addition operation.  RustPython crashes. 

test.py
```
mylist = []
n = 10000
print(eval("mylist"+"+mylist"*n))

```

Actual output:

> Segmentation fault (core dumped)

System:

> Ubuntu 16.04
> Rust Python 0.1.2 interpreter