⚡ 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;