clean: split code in multiple files

src/packet.h

@@ -0,0 +1,8 @@
+#pragma once
+
+#include <stddef.h>
+
+int packet_compare(const char *sent, const char *received, size_t packet_size);
+int packet_check(char *packet, size_t packet_size);
+int packet_update(char *packet, size_t payload_size);
+void *packet_create(size_t payload_size);