[eigen] Eigen with C++20

[Lawrence Ibarria <lawrence@xxxxxxxxxxxxxxxxxxx>]

Hi, 

We use Eigen heavily in our company and with C++20 now released and in most compilers I was interested in trying to put Eigen inside a c++ module. 

I have tried with Clang, but module support in Clang is rather lacking (as of 13.0.0.). 

With gcc, basic modules compile but Eigen has many issues. A main one is the complexities of merging C headers (intrinsics specifically) with modules, and otherwise I get compiler crashes (with g++-11). 

Has anyone looked yet into modules for Eigen? Any interest ? 

Thank you,