Dlahqr must not (and does not) write into wr and wi beyond ihi, so it only makes
sense to make the slices shorter. Dlaqr4 (the caller) will have the same restriction.
Previously, the zero value of the returned integer info was used to indicate
success. This is wrong because it is a 0-based index unlike the reference LAPACK
where it is a 1-based index and therefore 0 is available for such purpose. This
commit adds `converged bool` to the returned values from Dlahqr.
