Import std module

[davidhunter22]

These changes optionally all the library to be built and consumed using import std; rather than classic old style #include <meow>. This currently works for Visual Studio and Clang using libc++. It required the latest CMake 3.30 and Clang versions. This is not an attempt to create a module from the library, which would be nice to have, only to use the std module.

In CMake everything is guarded by a STRONG_TYPE_IMPORT_STD_LIBRARY variable. In code stuff is guarded by a macro of the same name. By default STRONG_TYPE_IMPORT_STD_LIBRARY is OFF is CMake so by default everything works exactly as now.

Many of the code changes are of the form

#if defined(STRONG_TYPE_IMPORT_STD_LIBRARY)
    import std;
#else
    #include <type_traits>
    #include <initializer_list>
    #include <utility>
#endif

Note both MSVC and clang can do an old style #include before an import std; but both give error when you do the reverse. For this reason in the tests the #include <catch2.hpp> had to be moved to be the first include as it does unguarded #include <meow>.

So as as user I can now #include strong_type.hpp with STRONG_TYPE_IMPORT_STD_LIBRARY defined and use import std;

One advantage of doing this is that the std module doesn't leak macros and global namespace things like uint32_t so you now know that you don't rely on things like that. I did fix a couple of cases where the library used ptrdiff_t from the global namespace. Note MSVC does have a bug where it does leak stuff into the GMF, see https://github.com/microsoft/STL/issues/1694 for details.

Anyway hope this is of interest, let me know if you want changes or documentation.