Constexpr domain source

Back in The constructor, we declared but did not implement a .get() function which returns a constexpr static instance of the domain. We implement this now:

// 100% constexpr instantiation
constexpr _file_io_error_domain file_io_error_domain;
inline constexpr const _file_io_error_domain &_file_io_error_domain::get()
{
  return file_io_error_domain;
}

As this is 100% constexpr, it can be (and is under optimisation) implemented entirely in the mind of the compiler with no run time representation.