Added functions are:
* json_string_nocheck()
* json_string_set_nocheck()
* json_object_set_nocheck()
* json_object_set_new_nocheck()
These functions don't check that their string argument is valid UTF-8,
but assume that the user has already performed the check.
It's now an error to try to add an object or array to itself. The
encoder checks for circular references and fails with an error status
if one is detected.
