splice() should take template instead of slist&

I have something like follows:

```cpp
class Parent : public slist_base_hook<>
{
    // virtual destructor, etc
};

class Child : public Parent
{};

slist<Parent> storage;
slist<Child> temporaries;
```

I need to move all elements from `temporaries` to `storage` at the end of a function. I've tried to use `splice()`, but it doesn't work because it expects a `slist` of the same type as `*this`.