* export public documented API
* don't export `namespace detail`
* add `std.h` into module
* add missing namespace qualification in `xchar.h`
* fix call to `detail::get_iterator` in `xchar.h`
* fix ambiguous overload of `detail::isfinite` in `chrono.h`
Introduce `FMT_BEGIN_DETAIL_NAMESPACE` and `FMT_END_DETAIL_NAMESPACE` for `namespace detail` sections embedded in that part of the code that contains all declarations that are exported from the module, i.e. which is enclosed by `FMT_MODULE_EXPORT_BEGIN` and `FMT_MODULE_EXPORT_END`. Given a correct implementation of C++20 modules, neither the name `fmt::detail` nor any of its contents will become visible outside of the module.
