Made connect_nodes_feedback!, a macro that allows building feedback
and sending an initial state so the graph doesn't deadlock.
Added a simple feedback case for testing.
In the sender that's automatically built by the create_node! macro,
the sender now holds a tuple containing the sender and a initial value
to send out when the node is instantiated by start_nodes! before the
node starts normal operation. This is used to handle feedback
scenarios while still keeping the channels as zero-size.
connect_nodes_feedback!, a macro that allows building feedback and sending an initial state so the graph doesn't deadlock.