include <iosfwd> instead of <iostream>

avoid bloat caused by <iostream> and std::cout and friends in apps where
iostream are not used.

test/src/unit-unicode.cpp

@@ -33,6 +33,7 @@ SOFTWARE.
 using nlohmann::json;
 
 #include <fstream>
+#include <iostream>
 
 extern size_t calls;
 size_t calls = 0;