Update Eigen to 3.2.3

Hello All,
This PR updates the Eigen library in `RcppEigen` to version 3.2.3. Previous patches specific to `RcppEigen` have been preserved and taken care of.
The [test script](https://github.com/RcppCore/rcpp-logs/blob/master/scripts/runRcppEigenDepends.r) has been run and all 25 packages passed the check.


Best,
Yixuan