Unicode support for *printf()

Hello all. Attached is the patch which adds Unicode support to *printf() functions stack. We (Andrei and me) made several assumptions that are worth mentioning: sprintf() and vsprintf(): - use runtime_encoding when dealing with Unicode data. printf() and vprintf(): - the result data is converted to output_encoding when formatting is done; - return _number of bytes_ outputted. fprintf() and vfprintf(): - use runtime_encoding, as all conversions are done by underlying streams API; - both functions return the number returned by streams API (which seems to be number of bytes). I did not run any benchmarks yet, but I don't expect it to cause any major slowdown. I would like to hear your comments before applying the patch, so don't hesitate to post them.