Returns the associated value of *string* as a null terminated UTF-8
encoded string, or *NULL* if *string* is not a JSON string.
+ The retuned value is read-only and must not be modified or freed by
+ the user. It is valid as long as *string* exists, i.e. as long as
+ its reference count has not dropped to zero.
+
.. function:: int json_string_set(const json_t *string, const char *value)
Sets the associated value of *string* to *value*. *value* must be a