Store backtrace to field of struct RException

Since the possible values of the backtrace are limited to `nil`, `RData`, and `RAray`, they are now stored as object pointers.

This change saves memory by eliminating the need to use instance variables for common exceptions.

ref. #2485

src/error.c

@@ -182,7 +182,8 @@ set_backtrace(mrb_state *mrb, mrb_value exc, mrb_value backtrace)
       p++;
     }
   }
-  mrb_iv_set(mrb, exc, MRB_SYM(backtrace), backtrace);
+  mrb_exc_ptr(exc)->backtrace = mrb_obj_ptr(backtrace);
+  mrb_field_write_barrier_value(mrb, mrb_basic_ptr(exc), backtrace);
 }
 
 static mrb_value