Negative Indexes with Slices (__getitem__)

[bsthorpe742]

Many objects do not support negative indexes with slices. The slicing behavior is handled with __getitem__.
Added support for negative index slicing in Bytes.__getitem__ in pull request #574. Bytes.__getitem__ could be used as an example for supporting negative slicing in other objects which support __getitem__.

[simsausaurabh]

voc/python/common/org/python/types/ByteArray.java is one among which supports __getitem__..
I am currently working on this to support negative slicing.
Thank you.