⚡ cmake compile time reduce using cotire

- Add prefix header
  - Include catch.hpp
  - Include json.hpp
    - Replace private with public for all json_unit files
- Move `unit.cpp` to an object library
- cotire issue: strip whitespace from CMAKE_INCLUDE_SYSTEM_FLAG_CXX

test/src/prefix.hpp

@@ -0,0 +1,7 @@
+#pragma once
+
+#include "catch.hpp"
+
+#define private public
+#include "json.hpp"
+using nlohmann::json;

test/src/unit-udt.cpp

@@ -26,16 +26,17 @@ OUT OF OR IN CONNECTION WITH THE SOFTWARE  OR THE USE OR OTHER DEALINGS IN THE
 SOFTWARE.
 */
 
-#include <array>
-#include <map>
-#include <string>
-#include <memory>
 #include "catch.hpp"
 
 #include "json.hpp"
 
 using nlohmann::json;
 
+#include <array>
+#include <map>
+#include <string>
+#include <memory>
+
 namespace udt
 {
 enum class country