Add the generational gc

include/mruby.h

@@ -122,6 +122,9 @@ typedef struct mrb_state {
   int gc_interval_ratio;
   int gc_step_ratio;
   int gc_disabled;
+  int gc_full;
+  int is_generational_gc_mode;
+  size_t majorgc_old_threshold;
   struct alloca_header *mems;
 
   mrb_sym symidx;