Suggestion: Package indirect.h and polymorphic.h inside a `xyz_value_types` directory

When installing value_types (cmake install or vcpkg), the header files `indirect.h` and `polymorphic.h` end up in the top level include directory. It could be better to group these inside an xyz_value_types subdirectory, so that clients would include them like `#include ` instead of `#include `. This is mostly a cosmetic topic, but it seems to be quite common practice for other libraries.