eigensolvers for Hessenberg matrices #858
Description
As mentioned in #625, it would be good to have specialized eigensolvers for upper-Hessenberg matrices (i.e. our UpperHessenberg) type.
In particular, Hessenberg factorization is the first step of LAPACK's Matrix eigensolver and we can skip it if the matrix is already upper Hessenberg. Indeed, LAPACK provides functions CHESQR and CHGEQZ etc for solving eigenproblems with Hessenberg matrices.