document flu_list dict functions
fix issue with flu_list_keys() and NULL keys
implement flu_list_dtrim()
get rid of flu_list_keys()

flu_list_dtrim() is enough

```
char **keys = flu_list_keys(l);
for (size_t i = 0; keys[i] != NULL; i++)
{
  void *val = flu_list_get(l, keys[i]); // O(n)
}
free(keys);
```

vs

```
flu_list *t = flu_list_dtrim(l);
for (flu_node *n = l->first; n != NULL; n = n->next)
{
  char *key = n->key;
  void *val = n->item;
}
flu_list_free(t);
```