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-readme.cpp

@@ -36,6 +36,7 @@ using nlohmann::json;
 #include <list>
 #include <unordered_map>
 #include <unordered_set>
+#include <iostream>
 
 TEST_CASE("README", "[hide]")
 {

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;