Changed the RData type field to 'const'

Within the mruby source code the 'type' field is either initialized, or used in const manner.
IMHO changing the type after it has been created might cause hard to track errors.

include/mruby/data.h

@@ -19,7 +19,7 @@ typedef struct mrb_data_type {
 struct RData {
   MRB_OBJECT_HEADER;
   struct iv_tbl *iv;
-  mrb_data_type *type;
+  const mrb_data_type *type;
   void *data;
 };