Add implementation for serialize() callback
Add key for EVENT
Add interface + stubs for serializer state helper(s)
Add implementation for serializer_context_new()
Add implementation for state_node_new()
Add implementation for serializer_context_push_state()
Add implementation for serializer_context_pop_state()
Add implementation for serializer_context_get_state()
Add helper for calculating number of written bytes
DRAFT for serialize_event()