Allow serializing SM state

It is useful in some applications to get a snapshot of the stream
management state for storage outside the process, in order to recover
from crashes and other things.  The get_sm_state already present is not
suitable for this because it returns a live object tied in to the
current context and such, and containing much unneeded internal-api
data.

Introduce xmpp_sm_state_set_callback which is called every time the
state changes with a serialized state, and a dual xmpp_sm_state_restore
which takes in this serialized state and sets up a new sm_state based on
that.

The serialization is considered opaque from the PoV of the API, but is
based on CBOR to facilitate easy debugging.

Makefile.am

@@ -190,6 +190,7 @@ STATIC_TESTS = \
 	tests/test_jid \
 	tests/test_ctx \
 	tests/test_send_queue \
+	tests/test_serialize_sm \
 	tests/test_string \
 	tests/test_resolver
 
@@ -285,6 +286,11 @@ tests_test_send_queue_CFLAGS = -I$(top_srcdir)/src
 tests_test_send_queue_LDADD = $(STROPHE_LIBS)
 tests_test_send_queue_LDFLAGS = -static
 
+tests_test_serialize_sm_SOURCES = tests/test_serialize_sm.c tests/test.c tests/test.h
+tests_test_serialize_sm_CFLAGS = -I$(top_srcdir)/src
+tests_test_serialize_sm_LDADD = $(STROPHE_LIBS)
+tests_test_serialize_sm_LDFLAGS = -static
+
 tests_test_snprintf_SOURCES = tests/test_snprintf.c
 tests_test_snprintf_CFLAGS = -I$(top_srcdir)/src