names declared to be exported at the point of introduction into a namespace *must not* be (re-)declared as exported later in the TU, e.g. when they are redeclared, defined, or specialized. [module.export]/6
Drive-by fix found during module testing: add a missing `detail::` name qualification
* 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`
* Don't explicitly delete copy ctor of dynamic_format_arg_store
Explicitly deleting the copy ctor causes the move constructor to not be
implicitly generated. This behaviour is different than what was in
v8.0.1 and causes code that relied on the move ctor of
dynamic_format_arg_store to break.
* Add test for dynamic_format_arg_store's move ctor
* include <memory>, don't use make_unique
