Upgrade to Eigen 5.0.1+

[eddelbuettel]

As discussed in #143 and #148 there is "moar Eigen to be had" and we have to move ahead of the 5.0.1 release to the current snapshot. Or at least so say @SteveBronder and @bgoodri.

So we plan to get 5.0.0 settled here, and then move on to 5.0.1+ which should hopefully be good enough for both stan and the rest of CRAN.

[eddelbuettel]

Given the time passed since the Eigen 5.0.0 release and the fact that am Eigen release is actually preferred given our relatively moderate RcppEigen release cadence it may simply be best to wait for a proper 5.0.2 release.