A little sptrsv cleanup before the main block effort

[jgfouca]

Change list:

1. Change typedefs to usings in sptrsv handle
2. Add block_size setting to handle (unused for now).
3. sptrsv_solve_impl: Wrap entire thing in a struct templated on Handle. This is a pattern I've used successfully in par_ilut and spiluk. The outer struct uses Handle to build up a set of useful types so they don't have to be defined over and over.