Remove on_{begin,end}_{string,number}

And add `done` arg to data callback

src/test.cpp

@@ -230,7 +230,7 @@ void doTestUnescapingUtf8(std::string const &escaped,
   CAPTURE(stride);
   auto c = noopCallbacks();
   std::string result;
-  c.on_string_data = +[](void *p, const char *buf, int len) {
+  c.on_string_data = +[](void *p, const char *buf, int len, int /*done*/) {
     auto &s = *(std::string *)p;
     s.append(buf, len);
   };