This is not intended to be a completed transition since it leaves the
libraries unusable to external client code, but rather as a step towards
use of math/rand/v2. This initial step allows repair of sequence change
failures without having to worry about API difference.
blas64: add length field N to Vector
Alongside, fix the implementation of mat.VecDense and mat.Diagonal, as well as other changes needed to fix this change.
Fixes#736.
