refactor: public headers cleanup

Public headers should be only the ones that a user may have a need to actually include in his/her code.

test/unit_test/static/ratio_test.cpp

@@ -20,7 +20,7 @@
 // OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
 // SOFTWARE.
 
-#include <units/ratio.h>
+#include <units/bits/ratio.h>
 
 namespace {