Class xstream::base64::istreambuf: xsgetn support
Class xstream::base64::ostreambuf: no xsputn support yet and flush doesn't flush partial data, for example suppose you wrote 2 bytes of a 3 byte chunk, it's possible to know the first 2 characters of the encoded data, but these are not flushed
Class xstream::fd::streambuf: implement seek operation when possible