Add a json_set_serializer() function to allow the string output of a json_object to be customized.

tests/test_set_serializer.expected

@@ -0,0 +1,9 @@
+my_object.to_string(standard)={ "abc": 12, "foo": "bar" }
+my_object.to_string(custom serializer)=Custom Output
+Next line of output should be from the custom freeit function:
+freeit, value=123
+my_object.to_string(standard)={ "abc": 12, "foo": "bar" }
+Check that the custom serializer isn't free'd until the last json_object_put:
+my_object.to_string(custom serializer)=Custom Output
+Next line of output should be from the custom freeit function:
+freeit, value=123