let all utf8/wide functions can be available just by including utf8/utf8.h

while here:
- remove utf8/utf8_stream.h, now we only need utf8/utf8.h to include
- add some new methods for converting from a utf8 stream to wide stream/string
- do some improvements in TextStream:
  - don't use temporary objects to convert utf8/wide
  - add put_stream() which takes TextStreamBase<> as its argument
    (uses an iterator instead of get_char() for reading)
  - let operator<<(const Space & space) serialize to json and not to Space

src/convert/baseparser.cpp

@@ -34,7 +34,6 @@
 
 #include "baseparser.h"
 #include "utf8/utf8.h"
-#include "utf8/utf8_stream.h"
 
 
 namespace pt