Rename machine_(u)int_t to mp_(u)int_t.

See discussion in issue #50.
diff --git a/py/objlist.h b/py/objlist.h
index b71ab4f..05a4c8d 100644
--- a/py/objlist.h
+++ b/py/objlist.h
@@ -26,7 +26,7 @@
 
 typedef struct _mp_obj_list_t {
     mp_obj_base_t base;
-    machine_uint_t alloc;
-    machine_uint_t len;
+    mp_uint_t alloc;
+    mp_uint_t len;
     mp_obj_t *items;
 } mp_obj_list_t;