This is an initial implementation of slice stepping and slice objects.
The __getslice__, __setslice__, and __delslice__ methods have been removed.
Slice expressions are now turned into slice objects with the OP_SLICE instruction.
Slice objects have a start, end, and step, all of which default to None.
Slice objects are passed to __getitem__, et al., as a normal parameter.
Support for slices in list.__getitem__, str.__getitem__, and bytes.__getitem__ has
been implemented.
