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

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

spa/tests/test-mixer.c

@@ -50,7 +50,7 @@ typedef struct {
   SpaPollItem poll;
 
   SpaSupport support[2];
-  unsigned int n_support;
+  uint32_t   n_support;
   SpaIDMap *map;
   SpaLog *log;
   SpaPoll data_loop;