Use types with known sizes where we can, easier to serialize

Add iterator for POD and use it to implement some demarshalling.

pinos/modules/module-protocol-native.c

@@ -141,7 +141,7 @@ connection_data (SpaSource *source,
   PinosConnection *conn = client->connection;
   uint8_t opcode;
   uint32_t id;
-  size_t size;
+  uint32_t size;
   PinosClient *c = client->client;
   void *message;