For example, now that we can import/export CSR and CSC (which incurs a copy, but may be more efficient than COO in general), we may want to consider moving some methods on x.ss to x if we like the API, find the functionality useful, and can implement it efficiently (say with numba in parallel).

Methods to consider:

- split
- concat
- head
- reshape
- scan / scan_rowwise / scan_columnwise
- selectk, selectk_rowwise / selectk_columnwise
- compactify / compactify_rowwise / compactify_columnwise