SpTRSV development steps

[learning-chip]

• Prepare sparse L factor as benchmark dataset
• Benchmark Intel SpMP trsv kernel as multi-threaded performance baseline for level-schedule algorithm
• Benchmark Kokkos sptrsv kernel as multi-threaded performance baseline for partitioned-inverse algorithm
• Internal code design for level-schedule algorithm. AMGCL's ILU solve is a relatively simple reference.
• Internal code design for partitioned-inverse algorithm
• High-level sptrsv API that is "in harmony with" existing GraphBLAS/ALP API and data structures
• End-to-end PCG benchmark with incomplete factorization preconditioner

[anyzelman]

@learning-chip I think at least the latter two steps are still wanted, correct? I suggest to target v0.8 for this one if that's OK -- still plenty of things planned to merge in v0.7:)