Implement row-wise splicing in c-contiguous 2d arrays

Even though general support for splicing is more complicated, it is
quite straightforward to allow row-wise splicing in c-contiguous 2d
arrays, which will cover a large number of use cases.
Added test for 2d slicing (and fixed small typo in GPUArray.__getitem__)
Added support for column-wise splicing of f-contiguous arrays
fixed some typos to make column-wise slicing of f-contiguous arrays work