Split backend and outgoing. Rename backend -> receive, client -> send.

send.h

@@ -0,0 +1,10 @@
+#pragma once
+
+#include "common.h"
+
+void send_init();
+struct serializer *send_get_serializer(char *);
+void send_add(int, struct serializer *);
+void send_add_wrapper(int, void *);
+void send_write(struct packet *);
+void send_print_usage();