Fix memory leaks in slist test

[f2404]

Fixes #40

@srdja As you can see, I had to manually free the memory whenever slist_remove(), slist_remove_first(), slist_remove_last(), slist_remove_at(), slist_replace_at(), or slist_iter_remove() were called.
Do you still think it won't be convenient to free the memory from within the slist code?

Also, there's still memory leak in slist_remove_all() which cannot (UP: can - see below) be resolved from the caller side:

    SList *list;
    slist_1234(&list);
    slist_remove_all(list);

It seems, slist_remove_all_free() should be called instead.

[srdja]

You're right. It is tedious to free removed elements by hand. I'm going to open a separate issue #45 to address this.