Avoid memcpy() on copying structure.

src/time.c

@@ -559,7 +559,7 @@ mrb_time_initialize_copy(mrb_state *mrb, mrb_value copy)
     DATA_PTR(copy) = mrb_malloc(mrb, sizeof(struct mrb_time));
     DATA_TYPE(copy) = &mrb_time_type;
   }
-  memcpy(DATA_PTR(copy), DATA_PTR(src), sizeof(struct mrb_time));
+  *(struct mrb_time *)DATA_PTR(copy) = *(struct mrb_time *)DATA_PTR(src);
   return copy;
 }