improvements to slice assignment

OK, this is very strange...

So this addresses #255 But now t520.py is failing.

In test t520.py  The expected output for 

```
print hash("hello")
```
is 113 -- which is forced in the first place.
But, after this modification I'm getting 115 when I run ./skulpt.py test

Now this is where I get confused.  If I run ./skulpt.py run test/run/t520.py  I get 3 for the hash("hello")

If I make a little  one line test program I get 3, if I run on skulpt.org I get 3, if I try on interactivepython.org I get 3.

Why would I get a different hash value for "hello" when  t520 is run in a series of tests versus as a standalone?  

@rixner, I see that Marie added t520 late last year... any thoughts on this and why the hash value had to be forced in this situation?