Add json_object_array_shrink() (and array_list_shrink()) and use it in json_tokener...

Add json_object_array_shrink() (and array_list_shrink()) and use it in json_tokener to minimize the amount of memory used.  This results in a 39%-50% reduction in memory use (peak RSS, peak heap usage) on the jc-bench benchmark and 9% shorter runtime.
Also add the json_object_new_array_ext, array_list_new2, and array_list_shrink functions.