Rewrite generation of _get_item_

[jpfeuffer]

First of all we should support get_item with e.g. strings for dict like objects.

But, even now for just size types it is not really correct

autowrap/autowrap/CodeGenerator.py

Line 1670 in 570a6f4

| cdef long _idx = $call_arg

It should first check for invalid indexes and only then convert to a fitting datatype. Currently we get signedness warnings because it is still a signed long type even after checking positivity.