improve fast parser for incremental parsing

related to davidhalter/jedi-vim#282
Interesting! You're right, this is definitely better semantically. Would be interesting if there's a performance gap...

...I just checked... 

```
>>> timeit.timeit('a = []; a[:] = []')
0.10172390937805176
>>> timeit.timeit('a = []; del a[:]')
0.0896310806274414
```
There is :-)