Move the scanned string into the value instead of copying it

The SAX interface documents that the string handed to json_sax::string() may
be moved from, and the DOM handlers already move the one handed to binary().
string() did not, so every string value was copy-constructed out of the
lexer's token buffer, which then kept the buffer alive at its high-water mark
until the next token overwrote it.

Moving hands that buffer to the new value instead. The allocation count is
unchanged - the value needed one either way - but the copy is gone.

  jeopardy      247.3 ms -> 240.9 ms  (-2.6%)
  citm_catalog    4.61 ms ->   4.48 ms (-2.8%)
  40k 30-char strings 7.80 ms -> 7.64 ms (-2.1%)

Note this deliberately does not extend to the object key. Moving the key
hands the lexer's buffer - sized for the largest token seen so far - to a key
that is usually short, so the next value has to grow a fresh buffer. Measured,
that costs 11.9% on a document of many small keys with longer values.

Signed-off-by: Niels Lohmann <mail@nlohmann.me>

include/nlohmann/detail/input/json_sax.hpp

@@ -222,7 +222,9 @@ class json_sax_dom_parser
 
     bool string(string_t& val)
     {
-        handle_value(val);
+        // the interface allows moving the value (see json_sax::string), which
+        // hands the lexer's buffer to the new value instead of copying it
+        handle_value(std::move(val));
         return true;
     }
 
@@ -532,7 +534,8 @@ class json_sax_dom_callback_parser
 
     bool string(string_t& val)
     {
-        handle_value(val);
+        // see json_sax_dom_parser::string()
+        handle_value(std::move(val));
         return true;
     }