Fix some issues with formatted_print:

* Add a va_list version which can be correctly used by ivprintf (fixes pycdas output)
* Use a `const char*` format parameter to avoid the need for clang warning workarounds
* Make better use of return values.

pyc_string.cpp

@@ -2,7 +2,6 @@
 #include "pyc_module.h"
 #include "data.h"
 #include <stdexcept>
-#include <ostream>
 
 static bool check_ascii(const std::string& data)
 {