Support C++20 Modules

[Twon]

Add support for modules once compiler support is there: https://accu.org/journals/overload/28/159/sidwell/#FN01

[Twon]

https://devblogs.microsoft.com/cppblog/using-cpp-modules-in-msvc-from-the-command-line-part-1/ https://devblogs.microsoft.com/cppblog/moving-a-project-to-cpp-named-modules/ https://devblogs.microsoft.com/cppblog/c-modules-conformance-improvements-with-msvc-in-visual-studio-2019-16-5/ https://devblogs.microsoft.com/cppblog/standard-c20-modules-support-with-msvc-in-visual-studio-2019-version-16-8/ https://devblogs.microsoft.com/cppblog/integrating-c-header-units-into-office-using-msvc-1-n/ https://devblogs.microsoft.com/cppblog/integrating-c-header-units-into-office-using-msvc-2-n/

[Twon]

https://tristanbrindle.com/posts/flux-modules-experiments

[jakeheke75]

CMake supporting modules with new compilers https://www.kitware.com/import-cmake-the-experiment-is-over/

[Twon]

Issue faced by boost on converting to modules: https://anarthal.github.io/cppblog/modules