For SS:GrB v9.0.0:

    track any changes to the v2.1 C API and release v9.0.0 when the
    v2.1 C API is released.

Future features:

    pack/unpack COO
    kernel fusion
    CUDA kernels
    fine-grain parallelism for dot-product based mxm, mxv, vxm,
        then add GxB_vxvt (outer product) and GxB_vtxv (inner product)
        (or call them GxB_outerProduct and GxB_innerProduct?)

    aggregators
    index binary ops
    GrB_extract with GrB_Vectors instead of (GrB_Index *) arrays for I and J

