* Test equality of different length strings.
* Add tab to json_pack whitespace test.
* Test json_sprintf with empty result and invalid UTF.
* Test json_get_alloc_funcs with NULL arguments.
* Test invalid arguments.
* Add test_chaos to test allocation failure code paths.
* Remove redundant json_is_string checks from json_string_equal and
json_string_copy. Both functions are static and can only be called
with a json string.
Fixes to issues found by test_chaos:
* Fix crash on OOM in pack_unpack.c:read_string().
* Unconditionally free string in string_create upon allocation failure.
Update load.c:parse_value() to reflect this. This resolves a leak on
allocation failure for pack_unpack.c:pack_string() and
value.c:json_sprintf().
Although not visible from CodeCoverage these changes significantly
increase branch coverage. Especially in src/value.c where we previously
covered 67.4% of branches and now cover 96.3% of branches.
