[visit-struct] Add new port

[zhihaoy]

https://github.com/cbeck88/visit_struct

The motivation for this port is that we do not have to lock ourselves
with Boost.Fusion, or Boost.Hana, and can benefit from some downstream
projects such as Configuru at the same time.

PS: I think it's okay to export targets with underscores in names? I feel
that it makes sense to match the include directory name.

[zhihaoy]

Underscore in CMake now.

The package visit-struct:x64-windows provides CMake targets:

    find_package(visit_struct CONFIG REQUIRED)
    target_link_libraries(main PRIVATE visit_struct::visit_struct)