Avoid including <iostream> in library code

Including iostream means introducing the static (global) constructors
and destructors for std::cin, std::cerr, and std::cout. That extra
init and fini code is undesirable when those streams are not actually
used.

Instead, we'll use the narrower includes for exactly what's needed,
i.e., <istream> or <ostream>.

src/token.h

@@ -8,7 +8,7 @@
 #endif
 
 #include "yaml-cpp/mark.h"
-#include <iostream>
+#include <ostream>
 #include <string>
 #include <vector>