Fix device slices for Buffer with fixed dimensionality in template.

[mcourteaux]

The device-slicing logic was broken on the C++ level, due to the fact that the buffer device-refcount for crops and slices assumes it was cropped/sliced from a buffer with an equal number of dimensions. This is incorrect for both cropping AND slicing:

• slicing from a buffer reduces the dimensionality (this is obvious)
• cropping from a sliced buffer inherits the "cropped_from" field from the sliced Buffer, which which has a higher dimensionality.

This was never tested for in the tests as they were all done with variable number of dimensions (i.e., AnyDims). Thus, I added a test that does a device slice on a Buffer<int, 3>.

The solution is to store that "cropped_from" Buffer as a generic Buffer<T, AnyDims>.

[mcourteaux]

Had a minor misunderstanding about the mechanism of InClassDimStorage. Should be good now, I believe.