base: Implement JSON string escaping

We encode JSON in the KDC's audit logs, and soon in bx509d's /get-tgts.
Therefore, we should be reasonable in terms of what we encode.

lib/base/version-script.map

@@ -146,6 +146,7 @@ HEIMDAL_BASE_1.0 {
 		heim_json_copy_serialize;
 		heim_json_create;
 		heim_json_create_with_bytes;
+		heim_json_eq;
 		heim_load_plugins;
 		heim_log;
 		heim_log_msg;