Thanks to Basile Starynkevitch for the suggestion and initial patch.
Thanks to Jonathan Landis and Deron Meranda for showing how this can
be utilized for implementing secure memory operations.
Encoding an empty array or object worked, but encoding it again
(possibly after adding some items) failed, because the visited flag
(used for detecting circular references) wasn't zeroed.
* Now that JSON_SORT_KEYS is implemented, take it into use with the
valid and valid-strip suites. This is to ensure that the tests
remain valid even if the string hash function is changed in the
future.
* Remove test_dump API test. Instead, implement the same tests more
elegantly in the encoding-flags suite.
