Clean up SM state serialization

* Refactor `xmpp_sm_state_restore()`

Add new internal functions to load u32 and string values. This also brings
length checks in order to verify we don't read more sm_state than there
potentially is and type checks to ensure we have the correct CBOR types.

Each element is added to the queue right after creation, so it won't leak
in case creating its content fails.

* Refactor `sm_state_serialize()`

* Store & restore ints as/from big endian, CBOR requires that

* Bring API names in line with the usual naming

* Auto-format sources

Signed-off-by: Steffen Jaeckel <s@jaeckel.eu>

.gitignore

@@ -65,6 +65,7 @@ tests/test_resolver
 tests/test_sasl
 tests/test_scram
 tests/test_send_queue
+tests/test_serialize_sm
 tests/test_sha1
 tests/test_sha256
 tests/test_sha512