* Add default implementation that throws exception.
* Implement `createROI` for `TBlob` and existing compound blobs.
* Use reference couting for TBlob memory buffer to prolong its life time for ROI blobs.
* Add private extension for ND ROI and use it as implementation detail for now:
* Add `DimSlice` and `TensorSlice` structures for generic ND ROI support.
* Add `make_roi_desc` function to create `TensorDesc` for ROI.